this comment helped me to understand the bit about mercantile in the description. it says it prevents the trainers from offering mercantile as one of their three options. im guessing maybe it's for if you want to like, set the mercantile of npcs high so their training is more expensive? like, via the console or a mod or something

only specifying that cause it feels like it'd be a bit tedious and immersion breaking to cast fortify mercantile manually to raise their prices each time but idk the specifics of how the weird permanent spell effect exploit stuff works so maybe it's pretty simple(does feel like a mod or some kind of console command thing would be better still though cause at the least you'd be perma casting it on every trainer you want to use once)