The source has been modified since the backup copy was saved.
Explanation
Rational Developer for System z has an autosave function
that creates a backup copy of editor files on the local system. This
warning indicates that the backup copy created by an autosave operation
is based on an earlier copy of the source. The backup copy will still
be opened in the editor.
Response
Use File > Save As to save the
backup to a different location and then use the compare tool to merge
changes from the backup with the source.