I knew that was coming.
In a nutshell, Flex Lists are a pre-approved portion of an invitation list. If a chapter ends up having a spectacular Recruitment and their Return Rates far exceed their expected past history averages, this list gives Panhellenic permission to not extend as many invitations as originally planned. If on the other hand a chapter has significantly lower than expected Return Rates, this is a list of PNMs that Panhellenic will go ahead and extend invitations to (more than the originally given number).