There are several reasons why a given understudy might be performing. If a role is covered by two people, one could be out sick or on vacation. If a performer is out of the show for COVID, meaning that they’ll miss at least one full week of performances, they might use multiple understudies to minimize exposure. (Recently when Lea Michele was out with COVID, both her standby Julie Benko and understudy Ephie Aardema went on.) Usually if someone is a standby, they are the first call when the listed performer is out, but beyond that, it’s usually situational.