Campagnolo 11 speed cassette, thin spacer question, what's it for?

[cmg]

I have several Campagnolo 11 speed cassettes some have a very thin spacer others do not. What is the spacer for? When is it used? the bikes that I've built with campy 11 spd aren't using the thin spacer. any clues?

[Mark McM]

I have several Campagnolo 11 speed cassettes some have a very thin spacer others do not. What is the spacer for? When is it used? the bikes that I've built with campy 11 spd aren't using the thin spacer. any clues?

If it is very thin (like almost paper thin), then that is the washer installed between the cassette and the lockring. See the Campagnolo 11 speed cassette user manual (first paragraph under "Assembly"):

https://www.campagnolo.com/media/files/035_2342_User_manual_11s_Sprockets_Campagnolo_Rev0 0_06_17.pdf