Field and storage settings on base fields
The base field definition implements the FieldDefinitionInterface and the FieldStorageDefinitionInterface, so it can act like a form API field, though it's not managed by the field module.

So the base field definition contains the settings of both, which you can see how the default values are set in BaseFieldDefinition::create():

  public static function create($type) {
    ...
    $default_settings = $field_type_manager-&gt;getDefaultStorageSettings($type) + $field_type_manager-&gt;getDefaultFieldSettings($type);
    $field_definition-&gt;itemDefinition-&gt;setSettings($default_settings);
    return $field_definition;
  }