This setting has a default of UOP when you register a room. You can however change the memolevel. A room memo is a memo sent to every registered nick in any of the room's lists or to everyone in a specified list such as AOP. This command lets you restrict who can send them to the people of your room. This is an Enterprise Edition feature.
Example:
memolevel aop
Memos are useful for announcing policies or information everyone in the room might need. For example, if you do not add AOPs often, you could send a memo to the entire room informing them of a new AOP. UOPs and such might memo information about someone who has been causing problems in the room. If the people generally will not mind getting memos from the room, you should set the access level low, but if you want to make sure that memos are only sent when they are actually important, you might want to set it higher and only use it rarely. See the memoserv commands send and sendto.