The documentation portal is undergoing maintenance. During this period, you can't create any new user credentials. If you do, you'll need to recreate the user credential after the maintenance period ends.
The Hub Server is the run-time component that manages core and common services for the Informatica MDM Hub. The Hub Server is a J2EE application, deployed on the application server, that orchestrates the data processing within the Hub Store, as well as integration with external applications.