What the heck does this icon mean? It looks like four boards nailed together!
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?
You must have accidentally created it while making the new assembly.
You can right click on it and select delete if you don't want it.
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