HP 1650B Programming Reference Manual page 296

Message Communication and System Functions
A-6
5. The header MACHINEl:ASSIGN 2,3 is an example of a compound
header. It places the parser in the machine subsystem until the < NL >
is encountered.
6. A colon preceding the command header returns you to the top of the
command tree.
HP 1650B/HP 1651B