Other writing directions
It’s outside of the scope of this document to address writing directions other than LTR or RTL, e.g. TTB (top to bottom).
Mixed use of let’s say both horizontal and vertical implicit text present at the same time would require magnitudes more work than just BiDi, and there’s no common need for that.
If it’s sufficient to pick a line orientation and line progression (as per the terminology used by ECMA) on a global basis, that is, retroactively applying let’s say top-down writing direction to the entire contents, then supporting terminals and applications should implement ECMA’s SPD sequence.
Details of such modes, e.g. rotating some glyphs, mirroring the mirrorable BiDi characters, transforming the box drawing characters, handling of scrollback buffer, shuffling of the arrow keys etc. would be subject to further research.