I think ST is correct (but we are speculating at this time).
Most likely, it enables a CVAR tied to a tag (say CVAR ForgeBooks) so when you perk into it, it sets a base value (Lockpicking Perk 1 = CVAR = 0.05, Engineering Perk 1 = CVAR=0.05).
If it adds to the chance, then they would stack.