X-default
The x-default value in hreflang markup designates the page served to users whose language matches none of the declared versions: a generic entry point or the default international version.
In practice
It completes the annotation set without replacing it: each hreflang group benefits from a defined x-default, often the English version or a language selector. Omitting it invalidates nothing, but leaves the engine to decide on its own for the rest of the world — better to choose your own default storefront.