I understood Proliferate to be an ability that allows you to, given any number of permanents and players of your choosing, add another counter of each existing counter type that is already on there. Like adding another poison counter to a player that already has one, or buffing a creature with a +1/+1 counter even more, or giving an opponent's creature with at least one -1/-1 counter another one.
There are some edge cases I'm unsure of:
- What if an attacker is exalted (+1/+1 until end of turn) and they proliferate during their attack? Do they get an additional +1/+1? Does it affect combat in the current turn? If they get an additional +1/+1, does it wear off at the end of the turn like the exalted buff?
- Similar situation with Hand of the Praetors: could you proliferate the +1/+1 buff it gives other creatures?