There are three columns config types that do similar things but still have subtle differences between them. These are the none type, the passthrough type and the user type.
The DataHandler discards values send for type none and never persists or updates them in the database.
Type none is the only type that does not necessarily need a database field.
Type none fields does have a default renderType in FormEngine that displays the value as readonly if a database field exists and the value can be formatted.
If no database field exists for none fields, an empty readonly input field is rendered by default.
Type none fields are designed to be not rendered at other places in the backend, for instance they can not be selected to be displayed in the list module "single table view" if everything has been configured correctly.
none field can be useful, if:
A "virtual" field that has no database representation is needed. A simple example could be a rendered map that receives values from
longitudefields but that needs to database representation itself.
A third party feeds the database field with a value and the value should be formatted and displayed in FormEngine. However, a input type with
readOnly=trueis usually better suited to do that.
Since the formatting part of
none fields can be done with
input type as well, most useful usage
none fields is a "virtual" field that is combined with a custom
renderType by an extension.
The TYPO3 core makes little or no use of
none fields itself.