Think of it as independent and dependant variables... If you were to graph the data what would be the x and what would be the y. That may not be straightforward in a lot of cases.
Here's a quick example, European Countries may have membership in many internation organizations (EU, Nato, etc.) and Organizations have many European Countries as members. In this case I would use Countries as Origin since a County can exist independently from the organization, whereas the organization is dependent upon a county belonging to it. Hope that helps.
