Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
The ASSGN command is also used to ignore (IGN) or unassign (UA) a
logical unit. An I/O operation for a logical unit that IGN status is effectively a NOP. When a logical unit is unassigned, its pointer to
the PUB table is removed.
Part 3. Conversational Monitor System (eMS) 288.1

April 1, 1981
288.2 IBM VM/370 Programmer's Guide