The AP21LV has an AUTO PROMPTING, alpha-numeric display and a 16 key, positive action keyboard that makes programming fast and simple. The AP21LVT is the identical except it has a Transfer Port. This port allows programs to be transferred from one AP21LVT to another, or to an AP21TR. Programs can be transferred from a computer with the optional AP21COMP Software and Adapter kit. The AP21TR is a plug-in relay module that is programmed from an AP21LVT, or a computer, using the AP21COMP Software. Once programmed, the AP21TR will provide the very same switching functions as the AP21LVT.