Root admins can send a global memo on the server or network to all the registered nicknames or to all the people who have a registered room.
Examples:
/msg operserv sendmemo founders All room owners.
/msg operserv sendmemo nicknames To all the users with a registered nickname.
This is a good way to send important information. If you are making any policy changes this can be used to announce them. If you have a large number of users having problems with something, you can send helpful information to educate them. For example, if you were to change the expiry time, you should probably memo your user base so they would know. However, memos are not a completely reliable way to send information, since some people may not read their memos.