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

 file-spec

    Specifies any part of a file specification (for example, a
    directory name or a file type) that the debugger is to use
    by default when searching for a command procedure. If you do
    not supply a full file specification, the debugger assumes
    SYS$DISK:[]DEBUG.COM as the default file specification for any
    missing field.

    You can specify a logical name that translates to a search list.
    In this case, the debugger processes the file specifications
    in the order they appear in the search list until the command
    procedure is found.