For skill-use leveling system, skill use cap means restriction on gained skill exp(i.e. the cap is the capping mechanism, not a literal limit).
1.Use once restriction: after target is hit/affected by skill once, it gives no subsequent exp.
2.On-hit restriction: only successful hit/effect gives exp, not every use of skill.
On-Chance restriction work similarly: if skill gives n% chance-to-evade, only when something is evaded successfully the skill exp is gained.
3.Area restriction: skill use in specific area is capped to N maximum exp, after which it will not give exp until the cap period wears off(e.g. one day). Used as anti-grinding mechanic.
4.Target-level restriction: skill exp is only gained with target level > skill level.i.e. you don't gain exp by blasting level 12 fireball at level 10 target.
5.PvP restriction: skill exp isn't awarded with PvP or with vastly reduced gain.
6.Item skill restriction: if item has a attribute with dynamic skill level, its use will only boost the item's skill level(exp gained goes to item skill level, not character global skill).