In a ballet performance of Jack and The Beanstalk that I was involved in, the cast and understudies were as follows:
Given the arrangement of the understudies and what parts are on at the same time, here's what could happen if a single person could not perform:
Why couldn't Megan just replace Jennifer? Don't ask me. It's not a complicated part. Maybe she was too small, but I think she was taller than Taylor at the time.
I don't even want to think about what would happen if two people got sick. (Lilani+Sarah, Alayna+Sarah, or Alayna+DeeDee would mean they would only have one imp.)