VMS Help DECnet-Plus, DCL Commands, SET, HOST, /MOP, Qualifiers, /BREAK=break-character  *Conan The Librarian (sorry for the slow response - running on an old VAX)

     Specifies the character which is to be recognized as a request
     to send a MOP Break protocol command to the remote system.
     Pressing the Ctrl key and this character transmits a MOP Break
     to the remote system.  The default character is \.