Mapping
class Mapping
Defines a mapping configuration element.
This object may contain any number and type of nested properties and each property key may have its own definition in the 'mapping' property of the configuration schema.
Properties in the configuration value that are not defined in the mapping will get the 'undefined' data type.
Read https://www.drupal.org/node/1905070 for more details about configuration schema, types and type resolution.