Library /sys$common/syshlp/lse$clihelp.hlb SET, MARK, Parameter  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 marker-name
    Specifies the name of the marker to be placed. For a marker name,
    you may use any combination of up to 21 alphanumeric characters,
    underscores, or dollar signs. If this marker name is already in
    use, the previous marker is canceled.