Each file, directory, or version considered for a comparison or merge is called a contributor. Typically, contributors are multiple versions of the same Rational® ClearCase® file or directory element.
Diff Merge selects one of the contributors to be the base contributor, which is the contributor against which all other contributors are compared when reporting differences.
If you are merging, Diff Merge also specifies a to-version contributor, which is the location where the merged output is stored.