The HAL System includes four types of control links (Level, Selector, Toggle, and Command). All controls participating in a control link must be the same type. Level controls, which are generally associated with volume, have a continuous series of states that are represented by a slider. Selector controls allow the end user to select an item from a list. Toggle controls allow you to switch between two states — on or off. A Command control has no state. It simply allows you to initiate an action.