Apple LaserWriter 12/640 PS - Manual (Page 208)

Loading labels in the multipurpose tray
You can load one sheet of labels at a time into the multipurpose tray for
manual printing.
Insert the sheet of labels into the paper cassette face up.
IMPORTANT When you print labels, use the multipurpose tray. (This helps
avoid paper jams and curling.)
Don’t attempt to print sheets that
have labels missing. This may cause a jam. Use only labels recommended
for use in laser printers.
208 Chapter 9