the bar facing x- does not matter, the cutting edge is still the same and the spindle must rotate in the same direction in order for cutting to occur irregardless of whether the tool is upside down or not.
With a right hand internal threading bar you will perform a right hand thread going from z0 to z+ (thread end) and a left hand thread going from z+ to z0
test it for yourself, take a screw with a right hand thread and hold it against an insert. Rotate it in the cutting direction and watch which way it moves in "z"
By feeding z in the opposite direction with this same direction of rotation we'll form a thread in the opposite direction.
I have a situation where I have a fairly long thread and I do not have enough guide bushing land. I flip my sp threading tool backwards and reset the core, giving me +.375~ more room to work with. I have to reverse the spindle, but since I also thread it backwards, going from z+ to z- I still create my right hand thread.
I went through this mind game before trying to create an OD thread on the sub spindle where all you have available for use are ID threading bars. Took me a while to wrap the relation around my head. One thing that makes it easy is ER = IL and IR = EL. So for my external right hand thread I use an internal left hand threading bar. This goes for inserts too. Cant find an IL? an ER will work. I know though that some manufacturers set their geometries differently for external vs internal threads.. but that is another story, and a nit picky one if you ask me. They work fine in my experience.
Confusing at first, but its really simple once you get your head around it.