Right, that didn't really help.

But editing the CR2 reminded me that each bone has a joint (and joint parameters) with its parent, and another joint with each child. So it would seem that jointx, jointz, and twisty are for the the joint with the parent bone (just one parent), while bone_2_twisty, bone_2_jointz, and bone_2_jointx are the joint with the child bone, the bone id being required because there can be multiple children (and the names without bone ids are already used for the joint with the parent anyway).

I'd guess that when you look at the joint parameters for a bone in the Setup room you're seeing the joint parameters for the joint between that bone and its parent, which appear in the CR2 as the  jointx, jointz, and twisty within that bone(bodypart/actor) but also as the bone_N_twisty, bone_N_jointz, and bone_N_jointx in the parent bone(bodypart/actor).

So while the x/y/zRotate of the second, third, etc bone in a figure bends the joint between it and its parent, the x/y/zRotate of the first bone in a Poser figure doesn't bend any joint, but simply rotates the bodypart. That's perfectly logical.

I think I need to insert a dummy bone. :)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).