There are a set number and type of magic items for each city based on its size. Then, you have the base limit. Any magic item up to that base limit has the 75% availability. The only items in the city available for purchase above that base limit are those set items. When, how, or even if, those set items get reset is up to the GM.
Magic items which fall under the 75% of a settlement's base value are the "commonly" available ones; your everyday items, say.
The items for the slots especially mentioned in the building's entries and for settlements in general are specifically produced by your dedicated magic item crafter in that town or as owner of that--they are there (and usually "filled") no matter what the town's value.
Ruyan.
Where is this in Ultimate Campaign? In PRD terms, anyway.
EDIT: NVM, under "Kingdom Turn Sequence" apparently. I'll BRB.
EDIT:
I'm not familiar with the building rules, but I am familiar with the regular magic item rolling rules (yet another edit: which, in this case, doesn't really enter the picture aside from the tables themselves). So is this the concern:
Your small city builds a building that has a medium slot. Small cities have a base value of 4 kgp, but medium magic items cost well above that, minimum. Thus, it is impossible to roll anything below the base value.
Ouch.
This was the result of two changes to the original kingdom rules being put in and having a consequence that many players have questioned, both to do with avoiding runaway kingdom wealth.
The first is that they made it so that you can no longer sell items in the city for BP, which was one of the reasons that the original kingdom rules from the Kingmaker AP could get runaway wealth.
The second was to prevent cities from having a chance of having any magic item at all, by limiting the maximum price of those items.
If either had been done, but not both, I doubt anyone would really question it. Both at once comes across as overkill on limiting the items available. Sean K Reynolds is on record as mentioning that you just can't please people: they complained about runaway wealth, and now they complain that they can't get expensive items.
My personal rule is to remove the second limitation. Allow the standard 75% for items below the settlement's GP limit, and fill slots with whatever comes up, even if it exceeds those limits.
It's a pretty bad flaw if the rules would have a situation like the one I described, where you're supposed to roll an item, but it's impossible not to exceed the base value. Is such a situation actually possible, or can, say, a small city not afford a building with a medium slot?
Edit: well, depending on item type, I guess. a lesser medium armor can fall below small city base value, but perhaps my example is bad. Still, if it's merely improbable that you'll roll a valid item, that can be worked around. It's impossible that causes a snag.
Hypothetically, a starting kingdom could ready a hex, build a settlement, and add a Caster's Tower as the first building, all of which would cost less than the 50 BP suggested starting treasury, and the settlement would have a population of 250, making it a small town with a 1,000 gp limit and 2 medium magic item slots.
However, those are random items of any type, and medium potions are completely legitimate items to end up with.
I still wonder if there are impossible combinations, but this discussion has inspired me to add another generation type to my random magic item generator, for UC building item slots, which work in reverse with respect to base values compared with typical random item generation. It can also report whether the combination of values you request is impossible.