This is the outer seal on the C1 piston for 1st, 2nd, 3rd, and 4th gears in 2006 to 2019 Allison 1000 6 Speed Transmissions.
It is also the outer seal on the C2 clutch piston for 4th, 5th, and 6th gear in 2006 to 2019 Allison 1000 6 speed Transmissions.
In either position, this seal prevents pressurized hydraulic fluid from leaking out of the piston inside the clutch.
