Right Weigh's 202-DDG-04 interior digital load scale has 4 internal air sensors and is designed for monitoring four height control valves (HCV's). This can be a combination of a dual HCV tractor, with two single HCV trailers, a single HCV truck, and 2 trailers with a total of 3 HCVs for both trailers, or it can also be used to measure an air-ride steer axle. The bottom line is that the number of height control valves you have total, should match the number of sensors the gauge has. On the display, you can setup an Estimated Steer Axle, Drive, Trailer A, and Trailer B depending on your configuration - as long as your truck and trailers consist of a total of 4 height control valves. After both an empty and loaded calibration has been performed at a certified ground scale for all axle groups attached, the gauge will reliably display the on-the-ground axle group weight in pounds or kilograms as well as connect to the Right Weigh app to view weights. When properly calibrated and used, Right Weigh's on-board load scales are the most valuable tools available for maintaining safe and legal axle weights. Corresponding Installation Kits (NOT INCLUDED): 101-SK Covers the installation from the back of the load scale to an axle group's air suspension system 301-QDK Provides the quick disconnect and air line needed when connecting a truck-mounted gauge to a trailer axle group Use a combination of these kits depending on where your height control valves are. For example, if you have a dual HCV drive, and two single HCV trailers, purchase 2 x 101-SK (drives) and 3 x 301-QDK (1 for the first trailer, and 2 to get back to the second trailer)