This option will set a specific mode on a client when they log onto your server. To view all the modes available for a client please refer to the user modes section listed in server commands for all users.
/as client Z-default set mode +ix
This becomes a user's starting modes. They can alter them as they choose, except in ways that conflict with a modelock. It is a good idea to start users with a general set of modes that most of them will like. This gets new users started well, and they won't change their modes until they know enough to do so deliberately. On large networks you will probably want to include +i and +x as part of the initial mode. These modes help protect people from spam or other annoyances. Other good modes to consider are +w, if your network tends to send useful information through wallops and +L if you prefer your users to start off with profanity filters.