Library /sys$common/syshlp/dbg$help.hlb DEBUG, SET, LOG, Parameters  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 file-spec

    Denotes the file specification of the log file. If you do
    not supply a full file specification, the debugger assumes
    SYS$DISK:[]DEBUG.LOG as the default file specification for any
    missing field.

    If you specify a version number and that version of the file
    already exists, the debugger writes to the file specified,
    appending the log of the debugging session onto the end of that
    file.