SharePoint's repository is the content database that resides in the SQL server. These databases house all the data for an organization. Organizations may require that users outside the company have access to a subset of this data. For example, vendors may wish to see if their invoices have been paid.
Another example at a large enterprise is hourly workers may see a different subset of data than salary workers. The data all resides in the same content database.
Appropriately architecting the taxonomy and authentication lead into providing two different URLs. The end user will put in the appropriate URL and be directed to the trimmed content associated with that URL.
This is the point of AAMs. This recipe ...