A dedicated reference host is not required to be available to perform host profile tasks.
When you create a host profile, you extract the configuration information from a specified ESXi reference host. In previous releases, vSphere required that the reference host was available for certain Host Profiles tasks, such as editing, importing, and exporting. From vSphere 6.0 and later, a dedicated reference host is no longer required to be available to perform these tasks. For host profile tasks that require a reference host, an ESXi host that is compatible to the host profile is assigned as a reference host.
Sometimes, a compatible host is not available to validate the host profile during these tasks. If you make small changes to the host profile, validation is skipped and a you see a warning indicating that no valid reference host is associated with the profile. You can then proceed and finish the task.
Due to the introduction of this feature, users can no longer edit or change the reference host from the vSphere Client. The reference host selection occurs at runtime, without notifying users, in the vCenter Server for on-going tasks.