On the ribbon, go to Tools > Setups > Jobs > RFC Types.
This screen allows you to set up your different Return from Customer types.
If you have ticked Return Type is required for Customer Returns in Options > Job > Customer Returns, it is always required for customer returns. You will have to select a RFC Type when adding a return from customer (via Jobs > Return from Customer).
This is a useful means of tracking why goods are being returned, and can form part of your quality record.
By using RFC Types correctly, you can monitor your returns by running a list on a specific Return from Customer Type over a given time period.