Currently, we define a class property in the content type model for specifying a concrete field type definition e.g.
class: info.magnolia.ui.form.field.definition.TextFieldDefinition

It will be (@see MGNLUI-3882) also possible to define a shortened version of a concrete field e.g.
fieldType: textField (depends on how its defined in FieldTypeRegistry)

All those ways above let's 2Bean mechanisms to instantiate a field definition for you and you get the type for free if it's not being set explicitly.
However, if you only set type property but leave class and in the future fieldType property empty, then 2Bean transformers does not know which class has to be instantiated.
In this issue, it's aimed to have a mechanism which eventually infers this information from 'type' property with some default mappings.