I created a split negative plane for the power layer. When I create artwork, the split line comes in as a negative line, invisible except where it crosses anti-pads. The line is on the AntiEtch-Power layer and I ran the Edit-Split Plane-Create command. All of the clearances and thermals are in the appropriate locations in relation to the split.
Check Xsection & Plot mode in Manufacture->artwork
Better you can define planes as +ve plane . it gives more flexibity on controls