This tool is used to compare two builds of the same font — usually the old one against a fixed export — so you can see exactly what changed before shipping it. It diffs the rendered text pixel by pixel, compares metadata and table sizes, diffs every glyph outline, and reports regressions such as dropped hinting or characters that are no longer mapped. Supports TTF, OTF, TTC, WOFF and WOFF2.
Checks each font on its own, then checks what changed between them. Every finding says what it breaks in practice and how to fix it — usually by re-exporting from Glyphs 3 with the right options, occasionally by patching the binary with fontTools.