The Host Unit Hours is a separate license only from licensing/business perspective. Host consume host units and when you are starting consuming above the limit (purchased host unit licenses) you can consume host unit overages which are the host unit hours.
So when you are consuming up to the number of purchased host units, you are not consuming any of the host unit hours. Any agent connected or reconnected after you hit the limit will consume host unit hours (if they are available to you).
If you are not reaching your host limit, you are not consuming host unit hours.
So if you have for example 10 HU, and 10 HUH purchased, then:
As Julius said the host unit hours are only consumed when HU are at 100%, it is true that you might see some HUH consumption, but this is not real consumption but rather a calculation of HU translated in HUH. So nothing to worry there. I agree it does confuse customers sometimes.