It is for a Layout Sketch - you can create these in the top level assembly to drive parts when you are working on an assembly with motion.
pretty handy tool . . .
Mmmkay ... But I created this assembly, and I didn't make a layout sketch. There is a part in the assembly that someone else made. It has what may be a layout sketch, but it's completely internal to that part - except that I mirrored the part. Is that why I'm getting the four boards?