When it comes to using game rules to simulate a fantasy setting, one thing that's always been in the background in Pathfinder has been the idea that certain monsters, types of armor, and so on are more common in some areas than in others. For instance, Pathfinder First Edition's rules for the Knowledge skill rely on the fact that some monsters are more commonly known, though it doesn't really define which are which. Meanwhile, there are circumstances such as the spell Echean's excellent enclosure from Rival Guide where knowledge of a spell is closely guarded by its creator (in this case, an evil 20th-level wizard).
To make it easier for players and GMs to engage in worldbuilding, whether playing in Golarion or your own setting, we've created a formalized framework for the Pathfinder Playtest—using the categories common, uncommon, rare, and unique—that you can use to help determine the tone of a setting, region, or adventure. These are relative terms; while we list suggested rarities for various rules elements, they naturally vary from place to place even within the same campaign setting. For instance, in the playtest, a longsword is listed as common, and a katana is listed as uncommon, but in a game focused around Japanese fantasy (or, in Golarion, Minkai or Minkai-influenced nations in Tian Xia), a katana would be common and a longsword might be uncommon.
Something is common if it's ubiquitous in its category, like any of the core races and core classes, longswords, fireball, bracers of armor, and the like. All characters can select common options without restriction.
Something is uncommon if it's a little rarer, but still possible to find or use if you are deeply interested in it. These options are a bit weirder, more complicated, or known to fewer people, so they haven't spread across the world as much. Many uncommon options explicitly become available to a character as they proceed along a path that teaches them about that option. For instance, all domain powers in the playtest are uncommon spells, but clerics are granted access to domain powers through their deities. Characters from a given region, ethnicity, religion, or other group in your world might gain access to uncommon options associated with it. To go back to our previous example, even in a game set in the Inner Sea region of Golarion, if your character hailed from Minkai in her background, you and the GM might decide that you should gain access to Eastern weapons instead of, or in addition to, Western ones.
Story events in your game are another way a character could gain access to uncommon options. For instance, Stephen previously mentioned in the blog on alchemical items that you would have to get the formula for drow sleep poison from the drow; it's an uncommon option. But in your campaign, if your alchemist was captured by the drow and forced to brew poisons for them, the GM might add drow sleep poison and other uncommon options to the formulas available to your alchemist! Uncommon options make amazing rewards to find in adventures, and they can be found at a much higher rate than rare options, since they are more common in the world.
Something is rare if it's extremely difficult to obtain without doing something special in-world to find it. This means that rare options involve interplay between the player and the GM, or are granted by the GM directly. There's no way to get access to these through choices in your character build alone. Rare options are spells known only to the ancient runelords, techniques passed down by the grandmaster of an ancient monastery in the Wall of Heaven mountains, golem-crafting secrets of the Jistka Imperium, and the like.
Something is unique if there's only one. Most artifacts are unique, as are certain monsters, like the Sandpoint Devil or Grendel. No artifacts appear in the Playtest Rulebook, so in the playtest only a few Doomsday Dawn monsters and hazards are unique.
Uses of Rarity
So how is this system useful to you?
Worldbuilding and Emulating Genres
First of all, your group can really alter the flavor and feel of the game by changing around the commonness of certain elements, allowing for a wide variety of genre play and settings through a relatively simple system. This is a big tool in your toolkit for worldbuilding. What would a world be like where the wizard was uncommon, or where all healing magic was rare? You can create a new subgenre or setting simply by shifting around the assumptions of what elements are common, uncommon, and rare. The rarities in the Playtest Rulebook are meant to show a good baseline for a typical Pathfinder campaign and make for a solid default if you're not straying to far from classic fantasy, but I can't wait until people start posting their modified schemes for all sorts of different concepts, from prehistoric to horror, and from low-magic (all magic is uncommon or rare) to super high fantasy mash-up (everything is common!).
Mechanical Diversity without Cognitive Overload
While some groups go for a kitchen sink approach to available options, many groups want to allow options from other books but are tend to stick to the core content because of the sheer mental load of learning, using, and preparing for all of those options, especially on the GM. With rarity, you have a framework for adding more material without just opening the fire hose: you start with common content (for a new group, probably the things labeled as common in the book) and you expand into mastering uncommon and rare content only as it appears in your game. If a particular rare spell hasn't shown up in the game, you don't have to worry about how it might interact with your character's build or your NPC's plot in the same way you might otherwise. A player can bring some desired uncommon or rare options to the GM, who can get a feel for the rules involved and decide when and how to introduce those options to the campaign. If the PC is interested in spells and items from ancient Osirion, perhaps the PCs find a new quest that takes them there, or is contacted by an Osirionologist NPC who's willing to trade her Osirian secrets for the PCs' help with a different adventure.
We love games with plenty of options, but we also want to consider fans who've told us that though they love new options, they started becoming overwhelmed by just how many options there are in Pathfinder First Edition. With the rarity system, you can enjoy the best of both worlds.
One thing that can be tough in Pathfinder First Edition is giving a reward to a PC whose player has already looked up all the options and bought or crafted all the items they really want, learned all the spells they really want, and so on, even if some of those items and spells really seemed like they wouldn't be available on the open market. Rarity allows a GM to give rewards that aren't easily available without needing to homebrew a brand new item or ability every time, and allows players who gain rarer options to feel special and important.
When you emerge from a Thassilonian tomb with a rare spell few have seen in millennia, wizards' guilds might start salivating over that knowledge. Will you keep it to yourself? Will you sell it to select wizards for a pretty penny? Will you spread the knowledge to all who desire it, possibly making the spell uncommon or even common in your setting? Or will you keep it to yourself to show off for the spellcasters you meet who have never heard of it? Only you can decide, giving you the power to make a permanent mark on the setting.
So how are you most excited to use the new rarity system?