If you are trying to add an additional Lookup field to the Staff object from the Appointment or Sale object, you may experience an Issue where the Related List of the Staff page is overridden and the Sales as Sales Rep 1 Related List data no longer appears on the page.
This is caused by the naming convention of the Child Relationship Name on the custom field you created.
The new custom field Recast_By__c has Child Relationship Name = 'Sales', and it is making conflict with managed field Sales_Rep_1__c where Child Relationship Name has the same value
Here is the value on the custom field:
Here is the value on the out of the box field for Sales Rep 1:
To avoid this conflict you simply need to re-name the Child Relationship Name on your Custom Field:
When the Lookup field has a unique Name that is not referenced in the managed Visualforce page, the Sales and Sales Rep 1 Related List on the Staff page will be restored.