VM/CMS CP DRAIN Command
Ends processing on a unit record I/O device after current file is finished
Priviledge Class D
Privledge Class D:
The DRAIN commands stops a printer or punch after the current processing file is completed. This must
be done before loading a UCS buffer and is normal procedure before shutdown of the system as well as
before changing class or other attributes. The device may be restarted with the START command.
Reference the detailed IBM manaul page here.