From the CRM side, if you have enabled the Sub Panel for the objects, the portal users will get the Sub Panel access for a specific object.
The portal users will get the listing records of the related object as per the relationship with that object and along with the contact Object will be shown into the subpanel list.
Sub-panel will be visible as per the configurations from the Customer Portal Roles.
If you have enabled the Add/Edit/Delete rights, the portal users can access under the Sub Panel as well.