Source

File: wp-includes/customize/class-wp-customize-selective-refresh.php

public function add_partial( $id, $args = array() ) {
		if ( $id instanceof WP_Customize_Partial ) {
			$partial = $id;
		} else {
			$class = 'WP_Customize_Partial';

			/** This filter is documented in wp-includes/customize/class-wp-customize-selective-refresh.php */
			$args = apply_filters( 'customize_dynamic_partial_args', $args, $id );

			/** This filter is documented in wp-includes/customize/class-wp-customize-selective-refresh.php */
			$class = apply_filters( 'customize_dynamic_partial_class', $class, $id, $args );

			$partial = new $class( $this, $id, $args );
		}

		$this->partials[ $partial->id ] = $partial;
		return $partial;
	}