For security reasons, there is a concept of approve hosts. The reason is that if some malicious user obtained a copy of the discovery ISO, he should not be allowed to simply register it to the install env and than having clusters installed on it automatically.

The behavior is like this:

1: if the host is added using BMC, it will be approved automatically

2: if the host is added using boot it yourself, the host will show up in an un-approved state. Such hosts can not be used to install clusters on

3: the user manually approves such hosts

4: now the hosts can be used to host clusters