[Stavis_L]

They are straight percentage reductions to the gem cost for rituals. So, for instance, if a Conjuration ritual to summon 20 Knights who say 'Ni' costs 10 Astral gems, and you've got a Conjuration 20 site, it will only cost 8 Astral gems when cast in that province. If the spell was instead an Enchantment spell with the same effect, it would not have its cost reduced. Note that Construction bonus sites affect forging costs in the same manner.

I've heard it said that it also affects fatigue for spells of the particular school when they are cast in battle as well, but I haven't confirmed that personally, and this is generally much less useful.