If Norton is scheduled for the fifth performance, which one of the following could be true?

Just want some clarification on how it's possible that L could be 4th when Norton is performing 5th. The rule states" Lalitha performs third only if Norton performs fifth." Does this mean that if she performs 3rd he must perform 5th or if he performs 5th, she must perform 3rd. Im very confused on how she could possibly perform 4th if he performs 5th.