So the Source and Destination Room are basically two sides of the same coin. Once you offset your character from the active room you call the active room, source room, and the room where your character is currently in or where the minimap displays her, destination room. This terminology is very important for doing skews. Below you can see how it looks like if your position is offset. The room with the portal is the source room and the one with the savepoint is the destination room.