If you select Edit|Swap with Clipboard when the Window clipboard holds textual data, then it is swapped with the current selection in the active file. That is, the selection is put on the clipboard as happens when you use Edit|Cut. The text previously held by the clipboard is then pasted into the text like Edit|Paste does.
This command is always invoked on whichever editor is showing the text cursor (vertical blinking bar), regardless of whether you use the main menu, a toolbar button, or a keyboard shortcut.
Edit menu
Edit|Cut
Edit|Copy
Edit|Paste
Block|Rectangular Selections