Until Magnolia 4.5 a field definition like :
/modules/standard-templating-kit/dialogs50/components/promos/stkPromo/formDefinition/tabs/tabTeaser/fields/hideTeaserImage
didn't generate a field but was used to remove the equivalent field form the extrended dialog:
Let say that you have

dialogs/a/form/tabs/tab/
with
field1
field2
field3

and
dialogs/b/form/tabs/tab/
extends = dialogs/a/form/tabs/tab/
with
field2
extends = override

In this case, the dialog b would have the field1 and field3.

field2
extends = override
is used to exclude the field from the extend.

Now with magnolia 5 this field is mapped to a ConfiguredFieldDefinition and this class has no real field implementation.

Currently in this case Node2BeanTransformerImpl.resolveType define that this node is a 'field', field is mapped to info.magnolia.ui.model.field.definition.FieldDefinition and based on the type mapping of the ui-admincentral.xml file
<type-mapping>
<type>info.magnolia.ui.model.field.definition.FieldDefinition</type>
<implementation>info.magnolia.ui.model.field.definition.ConfiguredFieldDefinition</implementation>
</type-mapping>
mapp to a ConfiguredFieldDefinition.