I really thought it was a coding issue, can only glamour the same thing you can equip or whatever. I no longer think that's the case and I feel each piece is coded separately as glamour, mostly because of a bug that ended up happening in shadowbringers. Certain jewelry items (ones I saw were High Allagan and GARO earrings) could not be glamoured to certain classes despite them being equipable by ALL classes. They were a part of my glamour plates for a long time and were suddenly un glamourable for some reason, yet I could take them out and equip them directly. This isn't proof that glamour is coded separately, but it seems it might be the case? I still have the images from when I was going to make the bug report.
Has the red X there for something that can't be glamoured to the current class.