When the cross-reference is bidirectional, the same label is associated with both directions.
To avoid confusion, the convention is to name the label in relation to the forward direction. For example, if a label is used to express a parent/child relationship between A and B, A being the parent and A linking to B, then the label should be named 'parent'. If B linked to A, then the label should be named 'child'.
The source document refers to the document containing the original cross-reference in its content. The target document refers to document being pointed at.