Do you mean in the MPE mod editor? It means that an ability is disabled by another ability's presence.

For example, you could make it so that a unit suffering "vertigo" can't use its Wall Climbing ability.

Oh, that's fantastic... I will try to mess a bit with the editor (and no, not the mpe, just the standard one with my own mod) so for example i can mask i.e. Holy resistance while the unit get holy immunity at gold rank.... That will be good!

Thanks a lot Sir!