It's best to distinguish between carving operations with a carving bit such as the 1/16" ballnose bit, and vector cuts such as is being done in this project with the 1/8" straight bit. Vector cuts include cutouts, centerline text, and Select Bit on the oval operation such as this project.
I have made many of these two-board lake map projects (approximately 22" x 44") using two 12" wide boards. The lakes area is all one large pattern; the text is centerline.
On my first attempt, I just left the centerline text that fell off the virtual board in place. The machine would air carve off the board in areas that the centerline text would fall. Why? Who knows. I learned to delete centerline text that fell off the board. Also, I placed centerline text on my projects such that the text would not fall on board edges (notice there is no text at the exact middle of this lake map). So no text half on and half off a board.
Vector operations can't be "masked" off - they must be removed to avoid this behavior.
Carvings (patterns) that fall off the board do not exhibit this behavior. They do not need to be masked off. However, it is good practice to not carve to the edge of the board, to allow the compression rollers a good grip. So, I do mask off strips top and bottom, and the ends for the 7" rule, on my projects. I generally use a sled with 1"rails, so the masked off area prevents carving on the rails. To create the mask, draw a rectangle and make it a zero depth carve region.