After the Drop - operation


hello there,
i have a little understanding problem in chapter 23 of your great book. On page 315 in the printed version don’t quite get how the following method works:

-(NSDragOperation) draggingSourceOperationMaskForLocal: (BOOL) isLocal{
return NSDragOperationCopy | NSDragOperationDelete;

I am new to Cocoa Programming but i guess that the returned operation depends from the given flag. Earlier in the chapter i read that the flag says whether the drop happened inside or outside my application which would mean that the letter would also be erased when dragging to TextEdit or Preview which actually doesn’t happen. But actually NSDragOperationDelete is only returned when i drag the letter into the trash. How does this work ? I can’t get it from the code. I’d greatly appreciate an explanation.

PS: sorry for my bad english, i am not a native speaker and just learning