Alert signs in font view

Hi guys, simple question ¿what indicate the forbidden sign in font view?
I know about the yellow warning, and the brow corner, ¿there are more?

The ‘forbidden’ sign says that this glyph will not be exported to the final .otf.

Thanks! but i’m still in doubt ¿there are more, or a list?

What would cause a glyph to not be exportable? How can it be fixed?

If you set it to not export. Incompatibility will export an empty glyph. Both is documented in the handbook.