When a model works for free it is normally a "time for print" or "trade for print" arrangement. This type of arrangement allows photographers and models to gain experience and promotional images.
The cost of hiring a model is often expensive for beginning photographers, just as acquiring head shots or promotional images can be expensive for aspiring models. The time for print arrangement is beneficial to both parties. This type of arrangement is a negotiated process, with no existing definition except that money is not exchanged. The use of these photos by the photographer does require a signed model release document.