VMS Help SCA, SCA Topics, Glossary, compilation_unit  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    A compilation unit is the smallest piece of source code that
    can be separately compiled. For example, in FORTRAN, this is
    a subroutine or function; in C, this is a single file. Some
    languages allow you to compile more than one compilation unit
    at once. Even if you compile more than one unit at a time, SCA
    considers the units to be separate.