I don't know how it functions internally but in the end result, the Secondary Control Layers for the stretchy chain, only functionally assigns a layer for the Middle Control.
So, the start/end controls are assigned by the Primary Control Layers, all the controls in between are assigned by the first bone's original layer, and finally the Middle control is assigned by the Secondary Control Layers.
It's implied that the middle control is a primary control, and all inbetween controls are secondary controls.
If nothing else, the label should be renamed to Middle Control Layers.
I don't know how it functions internally but in the end result, the Secondary Control Layers for the stretchy chain, only functionally assigns a layer for the Middle Control. So, the start/end controls are assigned by the Primary Control Layers, all the controls in between are assigned by the first bone's original layer, and finally the Middle control is assigned by the Secondary Control Layers.
It's implied that the middle control is a primary control, and all inbetween controls are secondary controls.
If nothing else, the label should be renamed to Middle Control Layers.