You plug in the ht. of your vehicle, also the width, and length.

If you desire, the unit will display your current altitude all the time. The trade off is that only one thing can be displayed in this particular box at a time. For instance, if you want the current time to be displayed all the time, you would have to select the box, then scroll down and pick current time from the menu. Altitude would go away and current time would replace it. There are about ten different things that can be displayed in this box. If you want to keep your current altitude showing but just want to check your current time, for instance, just select the box, scroll down to current time. The current time will be shown, just don't select it. Hit the return button and your altitude will be there.

It can be used in a car or an RV... you select which mode you want.