What the storefront controls•The payment methods offered in checkout.
•Storefront preferences such as tax-inclusive prices and "from" prices.
•Links out of the flow — gift cards, private event requests, terms and privacy — which point at that storefront’s domain.
•The storefront the booking is created against; the ID is sent with the booking, and the guest is sent to that storefront for the receipt page.
If you leave it outUnderstory resolves a storefront for the company on its own. That is fine when a company has a single storefront — set the attribute explicitly whenever there is more than one, otherwise the widget may pick up settings and links from the wrong one.
Where to find the IDIn Backoffice under Storefronts → Implementation, the generated embed snippet already contains data-storefront-id for the storefront you pick. The ID is also the one in the URL while editing a storefront: /storefronts/<storefront-id>/edit.