This is from the comptia certmaster:
Volume based: One license that permits a specified number of installations, for example, installation of the software on up to 100 computers
Subscription: Periodic cost; usually includes at least basic technical support, maintenance, and possibly upgrades
Neither answer sound particularly good here, but we at least know subscription costs can be based on usage (tiers of use) and Volume based is basically just buying in bulk. Volume based don't scale at all.
If you were like me and guessed that volume based licensing would be based on the amount of usage, because it sure sounds like it might be, CompTIA would like to remind you that you're an idiot.