WordPress at a glance
action is not described

customize_render_partials_before action-hook . WP 4.5.0

Fires immediately before partials are rendered.

Plugins may do things like call wp_enqueue_scripts() and gather a list of the scripts and styles which may get enqueued in the response.

Usage

add_action( 'customize_render_partials_before', 'action_function_name_9998', 10, 2 );
function action_function_name_9998( $this, $partials ){
	// action...
}
$this(WP_Customize_Selective_Refresh)
Selective refresh component.
$partials(array)
Placements' context data for the partials rendered in the request. The array is keyed by partial ID, with each item being an array of the placements' context data.

Where the hook is called

WP_Customize_Selective_Refresh::handle_render_partials_request()
wp-includes/customize/class-wp-customize-selective-refresh.php 359
do_action( 'customize_render_partials_before', $this, $partials );

Where the hook is used (in WP core)

Does not used.