These are abbreviations which stand for Long Edge Feeding and Short Edge Feeding. This is to indicate how a sheet of paper is being fed into the printer. If the paper, for instance an A4 in a A3 printer, is being fed with the short side entering the printer first, this is referred to as SEF (Short Edge First) The printing direction can be very important with some Transfer papers, always indicated by arrows on the back-print of the transfer.