Usage: SERVICES SET REGISTER <command> <enable|disable>

CHANS - Allows you to disable channel registration.
NICKS - Allows you to disable nickname registration.

These commands allow you to allow or disallow registration of nicknames and rooms. You can allow registration of just nicknames while not allowing people to register rooms.

/as help services set register <topic>

You could, in theory, only allow the registration of rooms. But since having a registered room requires a registered nickname, this is not a generally useful configuration. You can disallow both, allow both, or allow only registration of nicknames. The advantage of only allowing nick registration is that you can set up all the rooms you want in advance and then close registration. But nickname registration is much more important for an enjoyable chat experience. Nicknames give users a persistent identity on the chat network.