Mod Skill Use perk entries are applied to all XP even those gained from trainers, skill books and quest. Trainers were fixed for good in RRO-700, but the problem remains for skill books and quest rewards. The Skill Use Mult value on the AVIF record is correctly processed by the engine. This value can also be modified by scripts. Testing shows that script modifications persist across reloads but not restarts.
Requiem uses Mod Skill Use for the following purposes:
Adjusted skill rates
MCM option for reduced skill rates
Racial skill rates
Ancient Knowledge, Gift of Gab and Painful Regrets