The Direction type represents a direction in the Unicode bidirectional algorithm; not every value in this enumeration makes sense for every usage of Direction; for example, the return value of unichar_direction and find_base_dir cannot be WEAK_LTR or WEAK_RTL, since every character is either neutral or has a strong direction; on the other hand NEUTRAL doesn't make sense to pass to itemize_with_base_dir.
The TTB_LTR, TTB_RTL values come from an earlier interpretation of this enumeration as the writing direction of a block of text and are no longer used; See Gravity for how vertical text is handled in Pango.