If you have and 06 up auto transmission vehicle you have a transmission control module(TCM). Depending on transmission, year, vehicle the TCM will either be inside the transmission or out side usually located near the PCM.
Why is this important?
Simple, in order to tune for change tire size, TQ convertor, adjust shift points, firmness, tire size, you must send TCM with the PCM. With out the TCM we can not tune the transmission side on a mail order tune. If you do not send the TCM we can not tune the transmission for any changes.
If you are doing and email tune you simply select to read it just like the PCM. This will allow us to tune it via an email tune.
Note: 2017+ GM 8-Speed and 10-Speed Tranmission Control Modules require unlock via HP tuners GM TCM Unlock service before tuning. This means you physically mail your TCM to HPTuners so they can modify it. The other option is buying a new unlocked TCM from HPTuners. Once unlocked you still have to have 4 credits with an MPVI2 just for the transmission side additional to the unlock fee. 2020 A10 is still currently waiting for tuning/unlock options from HPTuners.