What's the purpose of the T-pose? I know you mentioned it's for calibration, but why a T-pose above any other calibration pose?
TL;DROlder motion capture software needed a T-pose to tell body markers apart, since arms at the sides could swap markers; now it isn't required but keeps files clean and marks the animation start.
So what the T-pose did typically in the olden days, back in the olden days of motion capture, you would have to have the actor go into a stiff T-pose because the software needed to be able to recognize the different markers on the actor's body and say, this is an arm, this is the upper arm, this is the lower arm, this is the hand, chest, and so on. When the arms are down to the side, a lot of times those markers can flip and get swapped for markers that may be on the hips or in the chest. So by kind of spreading your limbs out to the side, it helps the system. Nowadays, it's not required, but it's really nice. It's a nice way to just kind of keep your file clean. It also is like a good time marker saying, this is the beginning of the animation.