Preferred Level Updating
There may exist more than one version of an update, each applicable to different versions of the same module. For example, you may need one version of an update for an unmodified base source module and another version of that update if a licensed program modified the modules. The AUX file used to update a particular module must then be selected based on whether or not a licensed program modifies that module. The AUX files listing the updates applicable to modules modified by a licensed program are called preferred AUX files because they must be used if they exist rather than the mutually exclusive updates applicable to unmodified modules. Using this preferred AUX file concept, every module in a component can be assembled using the one CNTRL file applicable to a user's configuration.
A single AUX file entry in a CNTRL file can specify more than one file type. The first file type indicates a file that UPDATE uses only on one condition: the files that the second and subsequent file types indicate do not exist. If they do exist, this AUX file entry is ignored and no updating is done. The files that the second and subsequent file types indicate are preferred because UPDATE does not use the file that the first file type indicates. Usually, the preferred files appear later in the CNTRL file in a format that causes them to be used for updating.
UPDATE scans each CNTRL file entry until a preferred file type is found, until there are no more file types on the entry, or until a comment is found. (A character string less than four or more than eight characters is assumed to be a comment.)