WordPress at a glance
action is not described

woocommerce_cart_item_set_quantity action-hook . WC 3.6.0

Fired after qty has been changed.

Usage

add_action( 'woocommerce_cart_item_set_quantity', 'action_function_name_7649', 10, 3 );
function action_function_name_7649( $cart_item_key, $quantity, $this ){
	// action...
}
$cart_item_key(string)
contains the id of the cart item. This may be empty if the cart item does not exist any more.
$quantity(int)
contains the quantity of the item.
$this(WC_Cart)
Cart class.

Changelog

Since 3.6.0 Introduced.

Where the hook is called

WC_Cart::set_quantity()
woocommerce_cart_item_set_quantity
woocommerce/includes/class-wc-cart.php 1353
do_action( 'woocommerce_cart_item_set_quantity', $cart_item_key, $quantity, $this );

Where in WP core the hook is used WooCommerce

woocommerce/includes/class-wc-cart-session.php 58
add_action( 'woocommerce_cart_item_set_quantity', array( $this, 'persistent_cart_update' ) );