Warrant Officers are usually civilian contractors who are treated as being 'between' Officers and Enlisted Ratings, due to specialized skills. They are addressed as 'Sir' and given all respect due an officer by Enlisted Ratings.

In formal address, they are addressed as 'Warrant Officer Name." They perform 'specialized' duties that may fall 'between' the usual Starfleet jobs. An example would be a civilian contractor who's along for a mission, observing a piece of equipment undergoing 'field tests'. They are entitled to wear an Officer's uniform.