I don't know it I'm calling it the right thing: I mean when you are creating a color scheme and hit shift-command-p to get what the current selector is (e.g. "meta.structure.dictionary.json") so you can style it.
It appears briefly in the status bar, but it is sometimes too long to fit, and disappears after a few seconds.
In TextMate, I usually need to take a quick screen capture of the tooltip that appears.
Having it as text, ready to be pasted into a theme file would be perfect.