I would need your help please with a question where I am not sure. I have the Zwift Hub and use the CS-HG700 cassette 11-34 on it. Now I have the info from Zwift that I don´t need the spacer for 11 speed cassettes, but Shimano told me I have to use the spacer when I use this cassette on roadbike freehubs. Now I´m not sure what I have to do right. Maybe somebody has the same constellation and can help me here.
If you install the cassette without the spacer, it should be pretty obvious if the spacer is needed. When the lockring is tight, the cogs will be loose if you need the spacer. (I haven’t tested this cassette on a Zwift Hub.)
Without owning a Zwift hub myself, I would assume it comes with the freehub for Shimano Hyperglide 11-speed road cassettes. This freehub is wider than the original freehub for Hyperglide 8-, 9-, and 10-speed cassettes. You can directly mount the (real) road 11-, and 12-speed cassettes from Shimano. Some cassettes, the article mentions 11-34 11-speed cassettes from Shimano are originally mountain bike cassettes. These fit the original freehub but need a spacer for the current freehub. I think you need a 0.85mm spacer in this case.
Shimano designed these two 11 speed 11-34 cassettes to squeeze onto older Hyperglide10 freehub bodies that have been around since ~1991. The two largest cogs are concave, and their cupped design positions them slightly over the drive-side hub flange. This allows use on the millions of bikes out in the wild that came with HG10-compatible wheels.
Shimano introduced HG11 freehub bodies about ~2013 that are 1.8mm longer. The Zwift hub comes with an HG11-compatible driver and that 1.8 mm spacer is needed to take up the extra space.
Other 11 speed Shimano cassette models are 1.8mm wider and use a different model naming convention. CS-R7000 and CS-R8000 come in the narrower gear ranges (11-28, 11-30, 11-32 etc). The two largest cogs on these models aren’t big enough to do the cup-over-drive-flange trick, so they can fit only on the longer HG11 freehub drivers.
Hope that wasn’t too much bike nerdism, but that’s the complete answer.