WP_Customize_Setting::is_current_blog_previewed()
Method: Return true if the current site is not the same as the previewed site.
Source: wp-includes/class-wp-customize-setting.php:291
WP_Customize_Setting::update()
Method: Save the value of the setting, using the related API.
Source: wp-includes/class-wp-customize-setting.php:681
WP_Customize_Setting::preview()
Method: Add filters to supply the setting’s value when accessed.
Source: wp-includes/class-wp-customize-setting.php:319
customize_update_{$this->type}
Action Hook: Fires when the WP_Customize_Setting::update() method is called for settings not handled as theme_mods or options.
Source: wp-includes/class-wp-customize-setting.php:704