The Model MDT-2B 3-Channel garage door remotes block Coded Visor Transmitter is designed for use with Linear's access control products. The Model MDT-2B is a three-channel device. Pressing the transmitter's left or right front button sends channel one or two. Pressing the small button on the transmitter's top sends channel three. The top button can be hidden with the top button cover supplied. Also included are lithium batteries with a five-year service life and a visor clip. The transmitter is MegaCode format, which means each transmitter is factory preprogrammed with one of over 1,000,000 codes, virtually eliminating the possibility of code duplication. Because the receiver "learns" each specific code, no unauthorized person can gain access to the system by reprogramming a transmitter. Block coded transmitters are factory programmed to a sequential series of transmitter ID codes. When used with a Linear access controller, transmitters can be locally or remotely programmed into memory by entering the first and last codes of the block into the system. Facility codes can be selected to further customize the system. The ACT-21B is only sold in multiples of 10. You will need to enter the facility code and range you need. If you do not know the facility code or range of your remotes, please contact your installer.