Once set the field cannot be changed during that chat session. Network operators will still be able to change their fields as often as they would like.
/as field Comment set once on
This can be used to encourage people to use fields for descriptive purposes and not to just play with them. In theory someone could try to flood people with their field changes, but set once would prevent this. It does, however, force people to settle for fields with any mistakes they may have made upon initial setting.