Customers build a custom contact list (a live filter, e.g. everyone with a Beta Tester value) and want to set or add a contact attribute value, including adding one option to a multiselect attribute, for every contact in that list in one action. Today Contacts has no row selection or list-level bulk action; the only route is a CSV import by user ID, which replaces the whole multiselect value and cannot overwrite existing values for email-only matches.