public enum Direction
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.
If you are interested in text direction, you should really use fribidi directly. PangoDirection is only retained because it is used in some public apis.