The paste buffer is a temporary file created to store the data transferred from one place to another. Some functions (like Cut and Copy) write to the paste buffer while the others (like various Paste items) read from it.
The paste buffer is erased each time you exit DART Pro. If you want to save the contents of the paste buffer, choose the Save paste buffer command from the Edit menu. If you wish to audition its contents (provided that the material is sufficiently long), use the Play paste buffer command available from the Play menu.