By default, Diff Merge considers all contributors when it makes a comparison. You can 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.
If you are comparing elements and you make any changes to the listing, Diff Merge restarts the comparison. If you are merging, you cannot change which contributors are included in or excluded from the merge. (The check boxes and buttons are unavailable.)