I tried a Google search to find out what this setting does. Seems if I have it on or off the shadows still show between walls. Is there any use for this anyone knows about ? Thanks

"Don't split by lights"

Make a patch, double its verts. Press "v" select a set(2+) verts in a row or column. Then "Crtl shift x" will cut the patch.
Don't split by lights gets assigned to both sides of the patch.
Im guessing its to avoid different light values from patch to patch where it might not have the same amount of verts. I never use it manually but its in my .map on every patch I cut.