"PasteDiscard" is a front end token that pastes the contents of the Clipboard at the insertion point and clears the Clipboard after the paste operation is complete.
This token is equivalent to the menu command Edit Paste As Paste and Discard.
This is useful for conserving memory, particularly if the clipboard is large. If you want to retain the contents of the clipboard, you can use the front end token "PasteApply".
To execute a front end token, you typically evaluate a command of the form FrontEndExecute[FrontEndToken[token]].