Web pixel
Shopify's sandboxed way for apps to receive storefront and checkout events, such as checkout started and purchase, without editing the theme.
Shopify web pixels run in a sandbox that Shopify loads, and receive standard events: page views, product views, add to cart, checkout steps and purchases. Because Shopify's checkout is closed to theme scripts, a web pixel is how an app sees what happens there.
Pixels respect the store's customer privacy settings.
A testing app uses its web pixel to count how many visitors in each variant reach checkout and how many complete it.