I've never seen anyone mention these in any of their guides or anything else. It's where you exchange something like an alliance or a ceasefire or vassalage to get a territory far away from the homeland and usually use it to make money though it also can be used to expand in that region like when I was Milan and got Iasi, Burachest, and Durazzo and then used those territories to conquer the balkans. I was playing M/M