 The S383 silicon material of the Type I PCBM seal is more forgiving of pre-berth temperature differential between the two modules than the V835 fluorocarbon of the Type II.S383 is also more resistant to Atomic Oxygen encountered on orbit prior to berthing.The Type II was used to launch small elements in the Shuttle payload bay while bolted to an ACBM or to similar Flight Support Equipment because the V835 material is more resistant to the damaging effects of scrubbing under vibration.