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As I understand it, Mongolian can be written vertically (top-to-bottom); but unlike other vertical scripts the lines progress from left-to-right.
The ascent field in the OpenType VHEA table (version 1.0) indicates that the ascent is the distance from the centerline to the previous line's descent. This sounds like the ascent is leftwards-pointing.
But version 1.1 of VHEA changes the name (vertTypoAscender) and the semantics of the field, very carefully indicating that it extends from the center to the right...of the ideographic em-box. This description makes me suspicious; I would like to think that the correction was complete, but it appears to assume that vertical scripts are ideographic.
Following the version 1.1 definition means that the ascent describes the amount of space after (not before) a Mongolian line. I suppose that's fine, but it changes the way I think about ascent.