Mounted on the front end of the output shaft. The gear train consists of 2 planetary sets, that run on a sun gear that spins on the output shaft.
The whole planetary system is retained on the front of the output shaft with an external circlip. The selective circlip thickness’ are used to control the gear train end play.
The majority of planetary sets 3 gears in them. However HD units can have 4 gears.
Late model transmissions and after market gear sets can have 5 and 6 gears.
There are also different angles on the gears, so gear sets cannot usually be swapped around, unless both planetaries and the sun gear and the outer hubs are swapped all together.