- M:1
This means that only M number of mobs from the same type recognize you.
M could be set to very high number to make it look like all of the mobs will attack you when they see you.
example) goblin has 1:1 aggro type so only one will recognize and attack you at a time. Goblin archer has 2:1 aggro type so only two of them will attack you at a time.

- M+N:1
This means that only M number of mobs from the same type recognize you while there would be N supporting mobs.
example) white spiders in Tir cornail grave area has 1+2:1 aggro type. When you are surounded by white spiders in Tir cornail grave area, ony two of them will follow you. If you attack one of them, that one will switch to aggro state and one more white spider in no aggro state nearby will switch into the supporting mode.

If there are multiple types of mobs, each type will have their separate aggro chain. If you directly attack a particular mob, that mob will switch to aggro state regardless of which aggro type it has.