Absences - absence plan participation - required flag

Summary:

Please can someone explain how the required flag works on the eligibility profile section on the participation tab when an absence plan is created?

Content (required):

At present, our client has a large number of absence plans, and we need to evaluate whether they are all required, or whether we can slim down the number of plans to make maintenance easier to perform. We're not able to find any details about the eligibility profile section on the participation tab, about how an eligibility profile is handled, when it has the required flag ticked or unticked.

Version (include the version you are using, if applicable):