From the post:
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.