The f_depth field is used to indicate at which depth the character is inserted in the current frame. The depth defines the order in which objects are rendered. A higher number defines the objects most in the front. A smaller number indicates an object drawn further back. Although illegal, there can be any number of objects at any one depth value. However, the players may or may not properly render them. Usually the last added item at a given depth is drawn behind the previously added objects at that depth. Placing one object per depth is safer (legal) so you can be sure of the drawing order.
The f_objec_id_ref is a reference to an object previously defined with one of the Define... tags.