I have a release that only has a single piece of cover in a slim jewel case. The front has the usual title and cover art but there’s no separate piece of paper for the backside. The back part of the front cover art lists the track listing and copyright etc. Should the back part be labelled as “back” or “tray”? It should be mentioned that the disc is fully visible when viewed from the back of the packaging as there’s no proper back cover.
When I’ve had this, I’ve called it “booklet”.
This is how I have also looked at it. It is the start of the information pages included in the package.
Apart from Front and Back I treat most pages with words on as “booklet” unless they are an obvious poster.
Bit like those “booklets” that are just one sheet that folds out into 3 or 5 parts. They are just a single bit of paper which folds out wide. So that kinda matches this example - just no “folding out”.
I read “tray” as the actual plastic tray holding the CD and\or the artwork placed underneath the CD mounting when the jewel case is see through.