Autodesk 12812-051462-9011 - 3DS MAX 9 COM LEGACY SLM, Help File, Page: 1614 / 2577

Curve Conversion Between Quaternion and Euler
When an animation converts from one controller to the other, its integrity is
maintained as much as possible. This means that despite variations in key
interpolation results, the keyframed poses are preserved.
If you convert a quaternion rotation curve to Euler, the resulting tangent keys
are set to Smooth. If, on the other hand, you switch an Euler rotation curve
to quaternion, the bezier interpolation from the tangents is maintained as
much as possible. However, if that interpolation is too extreme and can not
be matched by TCB values, it is then reduced to fit under quaternion limits.
Extreme tangents on a Euler rotation curve
Using Euler animations with Layers, Motion Mixer, and Motion Flow
Both Euler tangents and quaternion TCB data are stored in each keyframe.
Therefore, even if you are using the Euler XYZ controller, you can still change
the profile of your curves using quaternion/TCB control values (on the
Key
Info rollout on page 4367).
If you add a new layer to an Euler controlled biped part, that layer is converted
into quaternion until it is collapsed back with the original layer. On collapse,
the curve changes back into Euler following the same conversion rules stated
earlier.
When clips containing Euler animations are brought into the Motion Mixer,
Euler tangents are preserved in the blending process. However, once they are
mixed together in a
mixdown on page 8045, the resulting animation becomes
quaternion.
Working with Biped Animation | 4295