Testing gave me the impression that it is 1% absolute value so if you have 17% base jam chance with 1 skill point in UAC jam chance you get 16%.
Now some mechs have UAC jam chance reduction of say 20% (warhawk arm).
If you pick 5x UAC jam chance reduction skill you have 25% overall reduction displayed, which would obviously reduce the overall jam chance to 0% if it was absolute.
I tested UAC on the mech and the UACs actually jammed but i had to fire like 20x nonstop.
So there is either just a UAC jam chance display bug or the UAC jam chance is really reduced too much based on the quirk.
Edited by Antares102, 02 March 2017 - 11:40 AM.