No slots are not used at all with this admission control type. you simply set aside the resources of a single host for failure, that is all you are doing. This indeed means the host will not be used during normal operations. If there is no apparent reason to do this I would recommend against it.
I actually described this in the free ebook that published recently: https://pages.rubrik.com/clustering-deep-dive-ebook.html?utm_campaign=authors
Thanks! Unfortunately its unclear why its set this way, will need to delve into the design docs again
Your book is very much on my to read list, thanks for the link