What does the small grey box to the right of some glyphs names mean?

[obiobik]

This thread might be the answer

What is the 'u' icon that appears on all glyphs?

A ‘u’ icon has appeared on all my glyphs. Not on other fonts. What is it? Why is it there? Is it important? How do I get rid of it? [Screenshot 2020-11-26 at 21.08.18] I see the ‘u’ icon is shown on your example under ‘Managing glyphs’ on the post Glyphs 3: Make Things You Love | Glyphs