Font means that we use numbers from the font file, if available, otherwise we fall back to the Manual numbers. In practice very few fonts actually define optical alignment, so there's no difference between Font and Manual. We hope that Affinity will encourage font designers to add this feature to their fonts. In the meantime you can set your own numbers in the same text style that sets the font face. (No change is a text style option which means the style either inherits its setting from the style its based on, or else does not affect the text it is applied to at all.) In theory it is possible to do automatic optical alignment (or kerning), based on the geometry of the glyphs. I think this is what Oval wants. We have no current plans for this.