Destroy is the destructor for the object instance. Destroy checks for an assigned object instance in FieldObject, and will free and release the object instance when the internal flag is set that indicates it is managed by the collection item. 

Destroy calls the inherited destructor prior to exiting from the method. 

An object reference created and assigned to FieldObject by an application are not freed in the destructor. The application must handle that task.

TIdFormDataField.Create 

TIdFormDataField.FieldObject