Thank you for getting back to us, and please excuse me for the delayed response.
When you're adding a custom field from the Custom fields for Module
(module being your particular module in the backend) Sitefinity does three things for you:
1. Creates a data field in the database that would store the field values (it's added as another column for the module items table)
2. Creates the field UI (so next time you edit/create/view an item the field will be displayed and you can enter and view the entered data)
3. Creates the backend configuration settings section which sets the field properties.
What you're replacing when specifying Custom interface for entering data is Step 2 - this controls how the field will look in the backend and frontend, however please note that your field will still use the default field type settings (Step 3). i.e if you selected field of type ShortText but applied custom interface for entering data to be ImageField, you will have an ImageField, but it will use the TextField settings. We are planning at releasing a sample of creating a field control from scratch and implementing custom confogurtation and data field, so it would cover all 3 steps above. The blog post should be out by the end of this week, in the meantime you can check this documentation section
for more information on FieldControls concept in Sitefinity.
All the best,
the Telerik team
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