The CRM is home tab of your portal. Here you can see all your clients, their status, and any custom fields you may have added.
Client details view
When you click on a client user, you can see the client details page. This page is a client-specific page that lets you quickly see messages, files, etc. (all the information from the modules you have enabled). In addition, on the right there is a summary page that lets you add custom fields.
With custom fields, you can store additional information like phone numbers addresses, etc. Once a custom field is added, it is available for all client users. Only internal admin users can add custom fields but all internal users can update the values of custom fields.