Generally uncommon and rare things just need a good story justification worked out between the player and GM. Some rare feats and items and things are really only useful in the specific adventure they were made for, but i think of uncommon/rare as just guidelines for GMs to determine how appropriate things are for the setting.
I hope I understood your question correctly.
I believe this is what you're looking for
https://paizo.com/threads/rzs5t5jd?Tarondors-2025-Guide-to-the-Pathfinder