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

    The suffixes precedence list is a list of  all  the  file  types  MMS
    recognizes,  arranged in a predetermined order.  MMS uses the list to
    decide which built-in rule to apply based on the order of  the  types
    in the list, the file type of the target currently being updated, and
    the existence of source files in the specified directory.

    You can alter the order of the suffixes precedence list by using  the
    .SUFFIXES directive.