VMS Help MMS, Description File, Dependency rules, Built-in Rules  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    MMS uses built-in rules when you omit the action line or the  source,
    or  both, from a dependency rule.  Built-in rules allow MMS to assume
    dependencies that are not stated  in  the  description  file  and  to
    perform  actions  necessary  to  update  the target.  To decide which
    built-in rule to apply to a dependency, MMS also  uses  the  suffixes
    precedence list.

    Built-in rules also allow you to  access  files  stored  in  OpenVMS,
    CMS,  FMS libraries and records stored in the Common Data Dictionary.