First is the replacement of the ECM, this would be units like the Thundermax.
Second would be units that reprogram the factory ECM, this would be units like Mastertune, SERT, SESPT
Third would be units that splice into the wireing, this would be units like a PC3, PC5, Fuel Pak
The Rev performance unit is using the second category and shipping the factory ECM back to you so instead of you tuning it they ship a canned tune to you in the ECM that you shipped to them.
So it's all in what your looking to do and what you expect from a unit. The first group means that you have removed th eFactory ECM and replaced it with something from another supplier. The second group allows you to make any tuning changes you need or take it to someone to do it for you. The Rev unit does this with a little twist in that you remove the ECM and ship it to them then they install the tune and ship it back to you with there tune in place. The third group allows you to make some changes after the ECM has made up what it wants done, this limits what they can and cannot do.