& parent rooms
 
  Parent rooms are a subset of zones. If a room is used as as zone object,
  it is a parent room (PR). PRs are like local "master" rooms. Exits in
  the PR are global to that zone, and $commands on objects in the PR are
  global to that zone. Parent rooms are only defined if globals are used.
  Parent rooms should only be used for very large zones which have a lot
  of global exits. Otherwise, a ZMO thing should be used, because command
  evaluation on a parent room is slower than command evaluation on a ZMO.
  Large numbers of parent rooms may slow down the game significantly.
  
  See ZONE OBJECTS and COMMAND EVALUATION for more information.