This is possible by using combination of Custom Subtab and Custom Record. The Custom Subtab will be on the same level as Relationships subtab. It can contain some single fields. The Custom Record will show a custom list of information and it will be visible under this new subtab as well. Following is the procedure how to establish it.
Creating Custom Record
By these steps we will create a Custom Record for storing the list information under the new subtab.
1. Navigate to Customization > Lists, Records, & Fields > Record Types > New
2. Provide Name of the Custom Record (example: Additional Details) and Save.
3. Still on the new Custom Record, under Fields subtab, click New Field. This first new field will be Customer information so the record could link to particular Customer record.
4. Provide Label: Customer, select Type: List/Record, select List/Record: Customer, leave Store Value checkbox checked, check Record is Parent checkbox (important detail), then Save.
5. Click New Field again. This new field will be for additional information. There might be created as many such fields as necessary.
6. Provide Label: (example: Custom information), select Type as needed or leave Free-Form Text, leave Store Value checkbox checked, check Show In List checkbox (so the information from this field would be visible on the list under your new subtab). Then Save.
7. If necessary, create more custom fields by repeating step 6.
Creating Custom Subtab
1. Navigate to Customization > Forms > Subtabs
2. In Custom Subtabs window click Entity subtab
3. Add new line here - Title: for example Additional Information
4. Do not select any Parent for this new subtab.
Customizing the Lead/Prospect Form
1. Navigate to any Lead/Prospect/Customer record in Edit mode
2. Click on "Custom" subtab – the new custom record list is already here (as sub-subtab "Additional Details"). Please note that it already includes buttons to add/attach records. It also contains Customize View button. We will now move this under your own subtab "Additional Information".
3. Above the title of the Lead/Prospect/Customer click Customize > Customize Form
4. Click Move Elements Between Subtabs button
5. Click Lists > Custom subtab
6. Find the new custom record here (Additional Details) and select the new Subtab on it: Additional Information
Note: This change will apply only for a particular custom Lead/Prospect/Customer Form.
Now when we navigate to a Lead/Prospect/Customer record in the customized Form, we will see the result. There will be visible the new Additional Information subtab on the same level as Relationships. When we click on it we will see the list information in it.
Note: It is also possible to move some custom fields under this new subtab and they will appear there.