What is the source plan type property?

Summary: At the risk of sounding like a total dummy, I need to ask what the Source Plan Type property does in EPBCS.

Content: I'm an EDMCS administrator standing up a new application for a new EPBCS instance and my go-to EPBCS administrator is out of office… I'm seeing the Source Plan Type property on the Account dimension and I understand that it's value must be a cube name in the target app, but in order to know what to select for each account I need to know what "Source Plan Type" even means. What does it DO? What are the consequences of choosing one cube versus another? Can anyone help me?!