Question on Identity Domains

Summary:

I'm returning to my OCI account after sometime. First time I logged in, I noticed that my tenancy didn't have the identity domains feature enabled.

Next time, I can see two identity domains to choose from.

Reading through the documentation, I'm aware of what the Default domain is. But what is the other one? The domain type is also confusing? I'm able to login to the cloud account using both of them.

Can someone please clarify?

Content (required):

Version (include the version you are using, if applicable):

Code Snippet (add any code snippets that support your topic, if applicable):