It could be the usable bed area for engraving/cutting. Cuts or scores give you more usable area of the bed, but engraving needs more space for the laser head to swing back and forth so it has some “braking” to do. In order for it not to hit the sides and without abrupt halts, that area is designated as unusable. Here’s an example of what I mean using your file.
On the bottom and right side are the no-no areas represented by diagonal lines. But when I turn your design
That line now extends to the left hand side as well. Like Xabbess has said, your design is a bit too tall for either direction, and I too use an artboard, only I set mine to 19x11. I chose those that ratio based on the Glowforge not being able to use the entire bed, even on a good day.
I’ve actually found that the left hand side is something like 5/8" unusable with engraves, the right hand side is closer to 1/2", and the bottom is about 1". So as long as I don’t fill my artboard to the brim, I am pretty successful with getting nice placement when doing large orders with a lot of prints going on one board.