Alternative URL validation is performed using the PrimitiveCollectionEditor. This client-side validation differentiates between collection editor properties and single-valued properties. When text is entered into the alternative URL field and saved, the updatedCollectionPropertyValue method runs the appropriate validation.

The validation determines if the URL entered in this field is unique by comparing them against the current site base and redirect URLs. Additionally, the URL is compared against all other sites’ site base URL, additional URL, and redirect URLs.