"Unique variant bonus amounts are described
by ability ratings, whether or not the stat is displayed by a bar. On armors, the boosts can
go up to a Maximum! (6) rating, although Ultra
(5) cannot be obtained. For weapons, boosts
can go up to a Very High (4) rating"
Thats from the wiki. But a lot of ppl say that weapon UV's can also stack to maximum. So whats the truth? :O
The UV itself cannot exceed "Very High" on a weapon, however since it stacks it can be pushed over. Example: Charge Time Reduction on a level 10 weapon is Medium (2) natively. If you have a UV of Charge Time Reduction Very High (4), the 2 will stack and the end result is Maximum (6).