By default, Diff Merge considers all contributors when making a comparison. To exclude a contributor pane from the comparison:
The first contributor in the list is the base contributor. If you are merging, the last contributor in the list is the to-version contributor.
When comparing, if you make any changes to the listing, Diff Merge restarts the comparison. When merging, you cannot change which contributors are included in or excluded from the merge. (The check boxes and buttons are unavailable.)