Direct thermal barcode printers require the use of heat activated thermal papers and films, but require no transfer ribbon. The print color is limited to black, unless special, preprinted direct thermal paper is used. The printing is not as crisp as that of thermal transfer ribbon printing, and over time the labels will darken. The main benefit is an overall lower cost, since ribbon is not required.
Thermal transfer barcode label printers require the use of a carbon ribbon which gets transferred onto the substrate via heat. The ribbons can be different colors, so you are not limited to printing only in black. The printing is very crisp and durable so this method is excellent for high density bar-codes and labels that require longevity. The other benefit is that thermal transfer printers can print on paper, film, and even foil substrates.
UPS packages require the use of labels on the package. The Zebra GC420D (replacement for the LP2844 printer) printer can print shipping labels for the following UPS thermal shipping label sizes (W x L): 4" x 6 1/4", 4 x 8, 4" x 8 1/4", and 4" x 11" labels.