External Clipboard controls how the Edit Paste menu command works in an X front end. (X only.)
This menu item provides a way to customize how the paste command works, since pasting is implemented differently in different X applications. If you set External Clipboard Clipboard, whatever was copied to the clipboard last (i.e., cut or copied from another application) will be pasted.
If you set External Clipboard Selection, the current selection is pasted. If you have trouble pasting from your favorite application, you might want to try setting External Clipboard to Selection instead of Clipboard.
The setting for this menu command can also be changed using the global option "XPasteFrom" in the Option Inspector.