Drupal 8 used to include both a 'default' and a 'undefined' type which both resolved to a PHP implementation called Property. The confusion in naming this concept three different things lead to misunderstandings as for its purpose. Therefore this is now unified under the schema type name 'undefined' and class name 'Undefined' and still universally used for the cases when configuration schema is not defined.
The Drupal 8 migration system uses configuration entities which may contain translatable pieces, such as the label of the migration itself or possibly some default data when creating new configuration. Also migrations may have a user interface in the future to create and adjust them, so while migrations may be written by hand mostly, they would also need to be saved safely with the Drupal API. These two indicate that they have configuration schemas. However some parts of migrations (according to the migration team) would be much harder to write by hand if refactored in a way that can be described by configuration schemas. Therefore we introduced a new 'ignore'/'Ignore' type to be used for this purpose, where parts of the migration structure can be avoided to be handled by schemas. We strongly suggest you use schema components other than the 'ignore' type to fully describe your configuration, this was introduced for the use case of migrations, where the configuration entity needs to be written by hand often.
