When navigating into or out of a repository collection, the source object is the one object to which all objects in the collection are related. The target object can be any one of the objects in the collection.
Don't confuse the terms source and target with the terms origin and destination. For any particular relationship, the source and target are dependent upon whether you are navigating via the collection of origin objects or the collection of destination objects. The origin and destination for the relationship are not dependent which collection is being navigated.