It’s more of a rigger/animator preference than modeler. The two poses have some tradeoffs to each other. The A pose is about making the characters bend and deform as naturally as possible. It reduces UV stretching around the high deformation stress points because the model’s arms are much closer to their resting position and don’t have to rotate as far. A poses have angled arms and often slightly bend the elbow as well, which means that the arm bones are a little more difficult to rig.
T poses require additional edge loops around the high deformation stress points like shoulders and hips in order to keep them from getting too ugly when those bones move too far. This is done to counteract the UV stretching mentioned above. The benefit of the T pose is that it’s much easier for the rigger to align the bones because the arms are in a straight line parallel to a major axis. Everything lines up much more easily and cleanly along the major axes without needing to do additional work.
Ultimately, the differences are small and up to the animation leadership’s preferences. Some prefer it one way, others prefer it the other, and a lot of times a project is just going with it because that’s how it was started and there’s been too much tech work done to switch to another and have to redo it all.
PS. These differences also apply to the rig’s default leg positions. Straight down? Angled? If angled, how big an angle? And so on…
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