It is a nested element. The content of the nested elements is updated via AJAX. By default, it calls on the wc_fragment_referesh JAVASCRIPT trigger. But you can change the trigger action based on the selected events.
Use case: Assume you want to display something after adding the product to the cart. You can use this element and the nested content of this element will show via AJAX.
We used this element inside the Mini Cart builder. We add the coupon form there. When a user adds/applies the coupon code to the cart, the content updater element calls and displays the applied coupon code on the Mini Cart section.
