The _lastrc built-in variable holds the return code of the last command issued. It also has a special attribute, message, that contains a human-readable string indicating what happened with the last executed statement.

The _rc built-in variable holds the highest-numbered return code that UDM has received (and placed in _lastrc from all processed commands during the current session.
 
_rc also can be set via the set command (see Setting Return Codes).

The _halton built-in variable specifies a return code that, if equaled or exceeded by the return code in _rc, causes UDM to exit.
 
Otherwise, when UDM exits, it returns the highest-numbered return code that it received to the UDM Manager.
 
_halton only can be set via the set command (see Setting Return Codes).