The AEM CAN Bus Expansion Module is designed to work with other AEMNet compatible products like the AEM CD-5 and CD-7 race dashes to allow additional inputs to be added. These inputs include fuel level sensor, high beams, and any other inputs that you do not have enough inputs for with the ECU you are using.
The AEM 6 Channel CAN Sensor Module is typically used with a CD-5 or CD-7 dash when you wish to display a fuel level sensor, and perhaps a few sensors that you don't have room for in your ECU.
The AEM 22 Channel CAN Sensor Module is typically used with a CD-5 or CD-7 dash when you wish to display a fuel level sensor, but have many more inputs you wish to add that you don't have room for in your ECU.
6 channel:
22 channel: