When is the cancel_url called by Stripe?

When calling the checkout session creation :

\Stripe\Stripe::setApiKey(env('STRIPE_SECRET'));
$checkout_session = \Stripe\Checkout\Session::create([
                                                       'line_items' => [
                                                                         [
                                                                           'price_data' => [
                                                                                    'currency' => $currency,
                                                                                    'product_data' => [
                                                                                        'name' => token_symbol() . ' Token',
                                                                                    ],
                                                                                    'unit_amount_decimal' => ($trnx_amount / $token)*100
                                                                            ],
                                                                            'quantity' => $token
                                                                         ]
                                                                      ],
                                                                      'mode' => 'payment',
                                                                      'success_url' => route('payment.stripe.success'),
                                                                      'cancel_url' => route('stripe.cancel')
                                                    ]);

The Stripe payment form does not have a cancel button even after clicking the "Pay" button. So what is the circumstance where the cancel_url is called ?

>Solution :

You customer will be directed to cancel_url when they click the back button on the Checkout page. Read the API reference for more details.