What is the ascertainability requirement for a class action?

ascertainable.' . . ."' [Citation.] The purpose of the ascertainability requirement is to ensure it is possible '"to give adequate notice to class members"' and '"to determine after the litigation has concluded who is barred from relitigating."' [Citation.] The ascertainability requirement is satisfied if 'the potential class members may be identified without unreasonable expense or time and given notice of the litigation, and the proposed class definition offers an objective means of identifying those persons who will be bound by the results of the litigation.'" (Sevidal v. Target Corp. (2010) 189 Cal.App.4th 905, 918-919 (Sevidal).)