Day-Elven, he's talking about normal units like beastmaster. that's what rexxar is. please play the campaign that comes with the game. he's talking about when you make a campaign style map like the rexxar campaign you have a set of maps that the player can move around on. so there are entrances and exits for the maps from one map to another right? and so how do those work? that's what Grantclan is asking.
assuming you kinda know what you're talking about, day-elven, do you mean file to import strings? if so your answer is just the tip of the real answer and not the whole thing. and even then, it's given wrong.
anyway...i think you need to go to the campaign settings. but i'm guessing you know that already! i haven't tried but i'm sure the solution is within there, the module campaign editor (F7)