VMS Help SOR Routines, SOR$SORT_MERGE, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 context

    OpenVMS usage:context
    type:         longword (unsigned)
    access:       modify
    mechanism:    by reference
    Value that distinguishes between multiple, concurrent SORT/MERGE
    operations. The context argument is the address of a longword
    containing the context value. When your program makes its first
    call to a SORT/MERGE routine for a particular sort or merge
    operation, the context longword must equal zero. SORT/MERGE
    then stores a value in the longword to identify the operation
    just initiated. When you make subsequent routine calls for the
    same operation, you must pass the context value supplied by
    SORT/MERGE.