Thermal Transfer Ribbon is also known as Fax Ribbon. It is used for thermal transfer fax machines.
Thermal fax machines are different from thermal transfer fax machines.
Thermal Fax Machines
Thermal fax machines heat thermal fax paper to make data appear on it.
Thermal fax paper consists of a paper layer, a binding layer (that also help spread heat on the paper) and a colorless layer that darkens with heat.
Thermal Transfer Fax Machines
Thermal transfer fax machines use thermal transfer ribbons as a middleman that contains the "ink" to transfer to ordinary paper. Its print head heats the fax ribbon to make data appear on the paper surface.
Thermal transfer ribbon or fax ribbon consists of a base polyester layer, a protective layer to prevent the ribbon from sticking to the print head and the "ink" layer.
The "ink" of fax ribbon can either be made with wax, resin or a combination of both. Wax is cheaper but can easily smudge or be scratched. Resin melts with heat and hardens upon cooling. However, it's more expensive. A combination of the two (aka wax/resin compound) gives the "ink" good quality at a good price.
Thermal transfer ribbon would eventually run out of "ink" and would need to be replaced.
Thermal transfer fax machines are often preferred over thermal fax machines. While they both have average printing quality compared to ink-jet fax machines, thermal transfer fax machine's fax ribbons require less care than thermal paper.