Steps to Display on Case Records the other Open Cases of the Customer

To add a sublist on the case record with other open cases for the customer follow the steps below.
Create the Saved Search:

1. Reports > Saved Searches > All Saved Searches > New.
2. Click on Case.
3. In the Criteria add Status is none of Closed, Pending-Close.
4. In the Available Filters tab add:
Customer : Internal ID (Customer Fields... > Internal ID)
5. Name the search and Save.

Create the Sublist:

1. Customization > Forms > Sublists > CRM tab
2. In a new row select the Saved Search from step 1 as the Search field.
3. Set a Label for the tab.
4. Set a Tab or leave blank to have sublist appear in custom tab.
5. Check box for Case.
6. Set Field to Company.
7. Save.