The window shows the current file attributes for the cursored file.
A LMB click in this window is the equivalent of a keyboard Shift B and will show the cursored file subject, if any, in the Message window.
A RMB click in this window is the equivalent of a keyboard Shift E and will activate the edit subject mode.
There is a simulated LED immediately below the window that will be yellow if the cursored file has a subject attached AND the file subjects are loaded. Otherwise, this LED is colored gray.