I cannot find, stepover apply method by plane / by part option any more in area milling operation.
Do they removed that feature method with NX update to NX 1926 from NX 12?
Go to File -> Utilities -> Customer Default
In the window select the sub category Operation under Manufacturing,
There you have the option to tick "Allow On-Plane Stepover Option in Area Milling" click apply save and close the NX. And the restart the NX you will find the on plane/on part option in area milling operation.
Grand Master Yoda