rem is related to the ch, though. It's not an awful compromise if a designer insists on tools like Figma. Sometimes just taking their work in rems and just removing the `r`s is enough to meet their designs and also have better relative units.
(The difference between rem and em is "root", ie, only using the top-most font size and sometimes the font-size before things like zoom and other similar adjustments. If a designer complains about the `r`s you removed, sometimes just showing them the difference at multiple zoom levels is convincing enough.)
(The difference between em and ch is em is wider. If a designer is always giving you weird decimals like 0.79em or 1.24em that may be an indicator that they themselves would be happier working and thinking in ch. em is the width of the capital M which is often the widest letter in most latin sections of fonts. ch is the width of the number 0 which is often the most "typical" width in the latin sections of fonts.)
(ETA: Both of which relate to the comment above complaining that both ch/em and others fall apart when dealing with non-latin scripts, because they are established as latin-centric.)
