Locking elevation while drawing a duct run avoids undesirable results in the design by restricting a duct run to a specific elevation. For example, you can avoid snapping to an object at a different elevation or trying to connect a horizontal run to a vertical segment.
You can turn on the elevation lock for a portion of a duct run, and turn it off for other portions where you want to use different elevations. If the lock is turned on, you cannot connect to an object on a different elevation. You must turn the elevation lock off before connecting the objects. By default, the lock is turned off each time you add duct.