This is something important that I think a lot of people don’t know about Bind Poses and Characterization poses:
You don’t have to change your character’s pose in the package you are exporting from.
Many people create characters with arms at a 45 degree angle down and that’s fine. You don’t need to model characters with their arms straight out (many people like the 45 degree angle for shoulder modelling reasons).
All you have to do is move your character into the preferred stance, once in Motionbuilder, and then Characterize it.
Also, if you notice that you didn’t Characterize it in the right pose, like you see the shoulders slouching too much, you just need to put the Character back into Stance Pose, check off the Characterize box in the Character Definition tab, make adjustments and then check the box back on.
Nowhere did I see this in any documentation, and it really caused me some pain early on until I figured it out.
Hope that helps people.