So... one of the things the Pathfinder game (and all OGL based systems, to a degree) is known for is the relative easiness of finding information online.
This presents an interesting dilemma for the GM and players alike.
While in the past you had to deal with physical books exclusively, and only used the rules and options in those specific books, websites such as the D20PFSRD include tons of information from virtually every Pathfinder rules compendium, including adventure paths, ever made.
So while at the table we only play with, let's say, the Core Rulebook. the Advanced Player's Guide, and the first bestiary, a player sometimes comes to me wanting to use a feat, a spell, an ability or an equipment from another book. And I, as GM, would sometimes want to use monsters from bestiaries other than the one I actively have on the table.
It's kind of a good problem to have, I admit, but where do you draw the line? Do you use anything and everything as long as it fits your campaign and/or characters, or do you simulate the experience of using physical books exclusively and only allow material from those books to be included in your game?
This is something I wondered about for a while now, but what prompted me to ask this now is that I'm introducing the players to the idea of traits, and thinking to myself if I should limit the players to the traits in the Advanced Player's Guide, or send them the enormous list of every available trait from the D20PFRSD...
When my group started playing Pathfinder we still thought in terms of physical books. Our first campaign only used the CRB, APG and Bestiary. With each successive campaign we introduced more books.
Nowadays the GM specifies which books (from the physical books we own) constitute the campaign's core material, and anything outside of those books (whether we own physical copies or not) is allowed or disallowed on a case by case basis.
We still use physical books in game and most of us are so familiar with them that we can find a particular rule more quickly with a physical book.
When I'm GMing I copy any material I allow from non-core books into a Word document and keep a physical copy of said document in my campaign folder. This means I have access to a physical copy of everything I need.
My usual rule is that I don't allow anything from a book that I don't own, and I'm often a lot more restrictive than that. I do this for the simple reason that the SRD/PRD is *HUGE*, and includes sources I've never read, and often (especially for 3PP stuff) never even heard of.
In my current homebrew campaign, we consider the CRB, APG, ARG, and Bestiary as "core," and I allow a handful of other books as secondary sources. I also reserve a few books (like Bestiaries 2+) for my use only. The ACG, UM, and UC are not currently available, because I'm still considering how much of them to allow, and most of my players haven't read any of that material anyway. (This blog post goes into a lot more detail on which sources are or aren't used in this campaign, and why.)
Occasionally, I (or one of my players) will find something in the SRD that I'll allow despite not having the book, but that approval is very much on a case by case basis. The only two examples that come to mind are: 1. I allowed some common, mundane gear (like bandoliers) from Ultimate Equipment before I owned that book, and 2. I made a Word document of all the religion traits in the SRD in order to help me choose which ones to assign to the many gods in my homebrew campaign.
I just handle it on a case-by-case basis. I have a relatively small list of pre-approved books, and if players want to use anything else they just need to ask me. I frequently use content from other books for my own purposes as GM, and I'm always consistent about it (if it's okay for the NPC's, it's okay for the PC's), but I do insist people get in the habit of running it past me so I don't get any surprises in the middle of a session.
Traits are one of those things that are usually fine, but there are a handful of really egregious ones out there. I'm not going to restrict access to the wealth of options out there because of a couple bad apples, though.
Everything published by Paizo is allowed at my tables. If it has been errata'd or changed at any point the players can use whichever version of the rule they want.
3rd party is strictly prohibited. I could care less if any of us has the book or has even heard of the feat before.
As a DM, my rule is: "if you ruin your own fun by being OP, then I'm not responsible."
If it's rules legal, I allow it. I own some 3PP stuff, specifically the DSP Psionics releases, both physical and digital.
A lot of my players are old school, and like to own physical books, and so do I, so I own a lot of them, but only the ones I feel are worth having at the table. To be fair though, there's not much difference in loading time on a phone vs. the time to thumb through a book at the table,
For my players, I disallow campaign traits from other campaigns for pretty obvious reasons. If I'm running a non-adventure path, I'll come up with my own campaign traits or offer a selection of campaign traits based on character backstories. I do require players to either adhere to the basis for a campaign trait or come up with an alternate reason for their character to have the effect of said trait. (I treat traits as devices for fleshing out an in-world character concept, not simply extra half-feats that tweak your numbers. IIRC, that was the intent behind traits as a mechanic.)
Other than that, I allow pretty much everything first-party with vanishingly few exceptions. Bans are usually due to the flavor of the campaign rather than any sense of brokenness (e.g., "laser rifles don't make sense in this setting").
As a GM picking out monsters? Everything is on the table--first party, third party, something I decided to just make up the night before. GM's prerogative (and responsibility).
(I treat traits as devices for fleshing out an in-world character concept, not simply extra half-feats that tweak your numbers. IIRC, that was the intent behind traits as a mechanic.)
To be honest, they're sort of hit and miss for that purpose. Using traits to add to class skills was a really good idea, since that's an area where mechanics and concept can often clash. Having an easy way to tack on a class skill alleviates a really common problem in that respect. The problem is that a lot of the traits are overly narrow and don't necessarily fit the concepts that might want them. This has gotten a bit better as more traits have been published, but it's often still a problem where the player will have a concept that clearly calls for a specific class skill but no trait fits for providing it.
Then there are issues with traits that should have just been feats to begin with. Magical Knack and Magical Lineage are the most obvious, but there have been others since then that simply do things that you can't replicate with feats and allow for mechanical builds that aren't otherwise possible. Then you have "filler" traits that are just obvious the best choices if you have nothing better to pick. Reactionary is the most common one, because +2 to initiative is something that's helpful for any character and is an obvious fallback. It's hard to criticize players for taking filler if nothing else appealed to them.
I think the best solution given the current state of traits is just letting players refluff them as they see fit. If you've got a reason for wanting stealth as a class skill, then you shouldn't need to go hunting for a trait that matches your concept. I'm a believer that mechanics and concept go hand-in-hand, so if a trait mechanically fits your character then fitting it in conceptually shouldn't be hard either.
I ended up allowing my players to officially use the traits from the APG but if they want to spend the time to research traits from other books, and they find something that fits their character pretty well (in their opinion), then I will consider it.
I should probably also point out that the characters are already level 4 (I am introducing the APG late into the campaign), so this whole idea of using traits to create your character from scratch is irrelevant to them.
I also feel that this "case by case basis" approach is probably the best one for me to use - I am already kind of using it "unofficially" - as when you find something cool in the SRD, and then realize it's not from Core but you'll use it anyway because it works... but I was just curious as to how other people approach this and it seems there are ways as the number of people using them...
We live in the age of abundant information... I'm sure it would've been easier if we just had a couple of books at the table. It's sifting through the information and finding the stuff that works for you which is the tricky and most time consuming part. I remember spending like two hours just reading about all the traps available in the game...
I allow anything published by Paizo unless I have specifically banned it
(Mythic, Synthesist Summoners)
I will allow 3rd party stuff if I have it available to read and after reading it consider it acceptable, notably things drawn from 2 3rd party publishers being combined I am very suspicious of as that is notorious for unintended synergies. I am also unlikely to add 3rd party stuff I can't fit into Herolab as I use it to make my life easier
Depending on the campaign I may ban other things for campaign specific reasons (such as high tech equipment, paladins or antipaladins)
I let people reflavour regional traits and otherwise slightly change them to fit the character concept accurately. I am unlikely to allow campaign traits from other AP's unless they are a very good fit for the character and consistent with the current game.