Field plugins provide a direct upgrade solution from Drupal 6/7 fields to Drupal 8/9 equivalents. They are responsible for handling the migration of custom fields and add processes to migrations for entities that have fields. When you write custom migrate field plugins they: usually extend Drupal\migrate_drupal\Plugin\migrate\field\FieldPluginBase. must be annotated with \Drupal\migrate_drupal\Annotation\MigrateField. must be in the namespace sub directory Plugin\migrate\field under the namespace of the module that defines them, for example MYMODULE/src/Plugin/migrate/field.