Attached Link: http://www.anticz.com/eularqua.htm

*Does Poser use Euler angles or Quaternions or both? Why don't they just use quaternions for all rotations (not just animation interpolation, as it appears by your statement)?*Just like probably any 3d application on this planet, Poser is internally using 4x4 matrices for rotations. The question is what values do you present to the user - and since most people think in 3 axis rotations rather than fourdimensional complex numbers, Euler angles are what the parm dials use. As soon as you have three text fields or parm dials with x, y and z rotation you will have to deal with rotation order, you can't avoid it.

Rotation order is not a Poser specific problem. Read the link wadams3 posted (I attached it to this post), it does a good job of explaining that.

Use the direct manipulation tool and get over it.