Here you decide what to see by a look-up from the No. field of a Purchase Line.
If this field has a check mark, a look-up will only show the Masters / Items available according to the Collection No. and/or Order Type of the Purchase Header.
If this field is empty, a look-up will show you all Masters / Items but when you enter one of the nos., you will get a message if the No. is not available according to the setup in the Purchase Header.