The Matrix by itself is just a group of a few vectors. The first three describe the position, and the other nine define each time three vectors with three "sub-vectors". All of that is ist based on the length of these 12 vectors. The Matrix has no concept of Head, Pan, and Bank (HPB)
The channels in the Timeline are not labeled to reflect that, as it would be confusing when the Rotation order would change, and then the naming and all values would need to be changed. That is not a given.
The node system is, so far, my impression, more of a developer platform for capsules and such. Solid knowledge like HBP v Matrix and why rotation order matters are expected. Again, that is my personal view.
Thanks for clearing it up, Larson Monkey; I searched for an Axis option inside the import options (hence my screen-shot) and even went back a few releases, just in case something would have changed.
The Axis Center's settings will be used for the Command "Center Axis To…".
To my understanding, it will not change the Instance object, but if you select "Use All Objects" in the Axis Center, it will affect the content of the Instance Object. I am not sure if that is the outcome you like to have. As the axis of the Instance Object is used then as an origin. I hope I got your question right. The Instance Object Axis manipulated by instanced points will change, but at that moment, move their content with them. So, if you use that Command again, the new position of the points, edges, or polygons will be used to position it. That can get messy.
From what I got, the All Object option in Axis Center will do that for you, but I'm unclear if the result will be to your liking.