/sys$common/syshlp/helplib.hlb PIPE, Description, Pipelines and TEEs, Using SYS$COMMAND  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    The SYS$COMMAND of a subprocess is normally the same as its
    SYS$INPUT (if no command procedures are involved). In a
    pipeline, however, the SYS$COMMAND of a subprocess is set to
    the SYS$COMMAND of the parent process instead of to the preceding
    pipe (which is the SYS$INPUT of the pipeline-segment command).