Help with MusicXML Positioning

I'm not sure if this is the right place for this, but the forum is closed,
so...

I can't seem to find the answer:

When we have a glyph, say an accent mark or a staccato, how do we know the
point on this glyph that default-x and default-y are originating from.  We
know that the default-x is relative to the left of the note, and default-y
is relative to the top staff line, but I don't know how these are relative
to the glyph.  Bottom left of the bounding box?  Center of the bounding
box?  Something else?

Sorry if it's in the documentation, I didn't see it.

.mjb