Outfits.

[Neruz]

I was referring to capping in it's basic form, saying that 'skills cannot go above X'.

Capping the number of skills one can acquire, like in Jackards example, or the total number of skill points one can have, like in Ultima Online, are both valid restrictions as they prevent one character getting everything, and as Almarian said, they're just variations on the concept of capping levels in a class based system.