[PvK]

If another mod uses the same unit numbers, they will mod each other if both mods are active in the same game. I have an old version of Vaettiheim installed and it merges Vaettiheim with Bogarus.

Example:

Underkings says, units X through X+6 are dwarf mages.
Bob's mod says, units X through X+9 are power rangers.

Result is no drawf mages, and power rangers with beards and axes... ;-)