Overlay is a method tocopy a drw view or sheet from another drw to your current one.
It is a read-only copy and will update if the source is updated. It copies all detail items as dims and baloons as displayed on the source.
Works similar as adding an extra model to your drw but you cant modify it.
Could be useful if you want to compare two similar designs or just need an external reference image.
I've never seen it in use, perhaps theres some benefits using it?
Regards
/Anders