VMS Help SCA, SET, LIBRARY, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 directory-spec[, . . . ]
    Specifies one or more directories, each of which comprises a
    separate SCA library. The list of libraries you specify replaces
    the current list of active libraries unless you specify an /AFTER
    or /BEFORE qualifier.