The domain of the user's email address is the validation check to affirm that the Admin Console organization may create the user.
"Just in time" (JIT) user provisioning refers to a configuration in the Adobe Admin Console that automatically creates a new user profile in the configured organization as the result of some configured trigger.
The below documentation describes the configuration process for creating a new user profile in the Acrobat Sign product profile when a user authenticates to Acrobat Sign using the organization's configured federated authentication solution.
The Hostname is inserted into the URL for the login page, providing a unique portal URL your users can authenticate through using your SSO solution.
This additional configuration adds the user to the selected product profile, entitling the user to access a service (in this case, Acrobat Sign):
Copy and share the URL in the Product access URL field with your internal users.
When used, this URL creates a new user in your Admin Console and entitles the user to the Acrobat Sign product profile.