A correlation domain is a group of resources that is created as a DX NetOps Spectrum container model. Condition Correlation assesses these resources collectively. This assessment is based on the rules in the policies that are applied to it. A domain can include any number of models of various model types and can have any number of policies applied. Therefore, when you select the resources in a domain, you are also deciding what is evaluated by the policy or policies that are applied to it.
You have multiple options for creating a correlation domain and populating it with resources. You can add resources on a per-resource basis. Or you can create a domain from a service or Global Collection model, which are entities that represent collections of resources.