Vertically Centred Icons

I feel that I should know the answer to this question, but I can’t find it at present…

The “Contents are centred vertically within the box” flag (bit 4) of an icon’s flags… what exactly does it do? I know that it has an effect for “text and sprite” icons, but for plain text icons or sprite icons it appears to have no effect. As one might expect from Acorn’s documentation, everything that I can find is quite vague on the specifics of when the flag is respected and when it isn’t.

So… does it apply to plain text icons and sprite icons, or does it only apply to “text and sprite” icons?