TECH II is STAND ALONE ECU. End of the line!
I have it on my Z. First hand experience here.
You may have an option to keep your stock ECU to take care of little things: Like idle. That is purely so to make your own life easier cuz techII could be a big fat b!tch to tune against sertain features.
But again - You can have all stock electronics guted out and TECHII will take care of everything.
And BTW: It has soooooo many features you will not know what the hell to do with all that!