Source

File: wp-includes/class-wp-customize-setting.php

protected function aggregate_multidimensional() {
		$id_base = $this->id_data['base'];
		if ( ! isset( self::$aggregated_multidimensionals[ $this->type ] ) ) {
			self::$aggregated_multidimensionals[ $this->type ] = array();
		}
		if ( ! isset( self::$aggregated_multidimensionals[ $this->type ][ $id_base ] ) ) {
			self::$aggregated_multidimensionals[ $this->type ][ $id_base ] = array(
				'previewed_instances'       => array(), // Calling preview() will add the $setting to the array.
				'preview_applied_instances' => array(), // Flags for which settings have had their values applied.
				'root_value'                => $this->get_root_value( array() ), // Root value for initial state, manipulated by preview and update calls.
			);
		}

		if ( ! empty( $this->id_data['keys'] ) ) {
			// Note the preview-applied flag is cleared at priority 9 to ensure it is cleared before a deferred-preview runs.
			add_action( "customize_post_value_set_{$this->id}", array( $this, '_clear_aggregated_multidimensional_preview_applied_flag' ), 9 );
			$this->is_multidimensional_aggregated = true;
		}
	}