From the document:

oVirt clusters have a compatibility version. The cluster compatibility version indicates the features of oVirt supported by all of the hosts in the cluster.

The cluster compatibility is set according to the version of the least capable host operating system in the cluster.

I didn't quite understand the last sentence.

How can the version of the cluster be determined based on the operating system version of the host?

Should the cluster compatibility version be determined based on the version of the oVirt engine on the host?