There's really no cold hard facts on this one, because everyones copy of OFP:R seems to be behaving differently.
For instance, the editor upgrade, which gets us placable OFP:R objects in the editor (for OFP:R that is) has to be in the \res\AddOns\ folder.
1.75 loads it's addons from \res\AddOns\
so if the addon has "required version = 1.75;" you have to put it in the res\AddOns.
But many addons don't require that.
But, to be on the safe side you should put everything in there.
1.46 addons can be run from there, but 1.75 addons can't be run from the original Addons folder.