Tony's post makes me realize that I don't understand this either.
Also, that I failed to complete my research.
So, the hierarchy is:
writing-mode (default ltr)
? Where does unicode-bidi fit in?
Unicode-bidi has values normal, embed, bidi-override.
The specific use of "direction" and "unicode-bidi" on inline objects is
to set the inline-progression-direction to be used by the Unicode BIDI
algorithm. This direction may override the inline-progression-direction
determined by the current writing-mode and the implicit direction
determined by the Unicode BIDI algorithm.
So direction overrides writing-mode?
To insure consistency with the "writing-mode" property, the "direction"
property is initialized to the value that sets the same
inline-progression-direction as is set by the "writing-mode" property
So direction is restricted to inlines? ie ltr or rtl? not, block-progression-
I guess I can conclude that this area is more complex that I thought
and that I'm at the end of my expertise here.