User Story:

Using late binding, once an infra env is created, the agent service running on the hosts registers the host with assisted service and become discoverable. At this stage, a host is known but is unbound to the cluster.

"status": "known-unbound"
"status_info": "Host is ready to be bound to a cluster"

Once the cluster object created, bind the host to the infraenv. A cluster ID is also required.
API to use is

curl -X POST 
http://<RENDEZVOUS_IP>:3001/api/assisted-install/v2/infra-envs/<INFRA_ENV_ID>/hosts/<HOST_ID>/actions/bind   
-H 'accept: application/json'   
-H 'Content-Type: application/json'   
 -d "{\"cluster_id\":<CLUSTER_ID>}"

When successful, the host will be bound. After binding the host, cluster will be ready for installation after a while once the validations are passed.

"status": "known"
"status_info": "Host is ready to be installed"

Hosts are always part of an InfraEnv. The late binding is to associate them with a cluster when the InfraEnv itself was not bound to a cluster.