What is the purpose of host rebuild reserve in vSAN?

The host rebuild reserve is a feature that allows vSAN to reserve space in the cluster for vSAN to be able to repair in case of a single host failure. This reservation is set to one host worth of capacity, which means that if one host in the vSAN cluster fails and no longer contributes storage, there is still sufficient capacity remaining in the cluster to rebuild and re-protect all vSAN objects. This feature prevents the creation of new VMs or powering on VMs if such operations consume the reserved space. By default, the host rebuild reserve is disabled, but it can be enabled in the vSAN Services configuration. The other options are not related to the hostrebuild reserve. References: vSAN Capacity Management in v7.0U1; Configure Reserved Capacity