The abbreviation PTO stands for Power Take-Off. There are three types of PTO-controlled models that are used in tractors including the transmission, live and independent.
Transmission PTO tractors are the oldest of the three types. In these models, the PTO shaft is connected directly to the transmission. These models require an overrunning clutch to prevent the tractor from continuing to move after using the transmission shaft.
In comparison, live PTO models use a two-clutch system that shuts down the transmission and the PTO separately depending on whether or not the clutch is fully pressed. An independent model has the PTO and transmission separated by having a shaft for operation of each.