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

 prompt-parameter

    Specifies the new prompt string. If the string contains spaces,
    semicolons (;),  or lowercase characters, you must enclose it in
    quotation marks (")  or apostrophes ('). If you do not specify a
    string, the current prompt string remains unchanged.

    By default, the prompt string is DBG> when debugging a single
    process program.

    By default, when debuggging a multiprocess program, the prompt
    string is the name of the current process set followed by a right
    angle bracket (>). You should not use the SET PROMPT command when
    debugging multiprocess programs.