Kickr Core and 11 speed Ultegra, do I use the spacer?

Do I need the thin spacer behind the cassette? Is that just for when you’re using a 10 speed cassette?

The spacer is for 10 speed cassette.

True for a typical road cassette.

But for the few people using an 11-34 Shimano road cassette, the need to use the spacer. That’s because this cassette uses the MTB cassette mount design.