You can either Cut (<Ctrl+X>) or Copy (<Ctrl+C>) the selection to the buffer. When you perform a cut, all selected subjects are copied into the buffer, and the selection is removed from the diagram (including the unselected edges that are adjacent to the selected nodes). When you perform a copy, no subjects are removed from a diagram, but a copy of the selection is made and put into the buffer. Both Cut and Copy copy the unselected nodes that are connected by selected edges into the paste buffer.
Pasting means that the contents of the paste buffer is being copied into the diagram. When you perform a Paste (<Ctrl+Y>) command, a stippled box is shown that has the size of the pasted area which is attached to the mouse pointer near its top-left corner. When you click button-1, the subjects in the paste buffer are copied into the diagram. The Append Diagram command is also implemented as a paste command. When you want to abort pasting while you are moving the paste box, you have to click button-2.
You can paste the same contents of the paste buffer more than once because always a copy of the contents is being made. Pasted nodes and edges are new nodes and edges, not duplicates of existing nodes or edges. The paste buffer remains intact when you load another diagram, so it is possible to cut and paste between different diagrams (although in the same editor).