Many of AP’s rotation matrix functions have formal parameters in RPY (123) order. But are the functions actually composing matrices in this order, or in normal aeronautical YPR (321) order?

The same question goes for AP’s quaternion convention.

I’ve studied both the matrix and quaternion libraries and am finding it difficult to make a definite determination. Please could someone advise?

the default is 321 order, but we have some functions that have a specific different order, for example, we have from_euler312 and to_euler312 on Matrix3 which we use in some special cases. All others are 321