Need an explanation of how Primary Flag works on Assignment Schedules

Summary:

We have a situation where we have employees with a single worker primary assignment, They have had work schedules changes relating to specific periods.
In this case I believe the Primary Flag should always be Yes as it addresses different time periods.

We also have a situation where we have multiple worker assignments, where each assignment is linked to a work schedule and different accrual plan.
Employee chooses which assignment to book absence against. I believe the Primary Flag should be set to Yes on each work schedule for each worker assignment. Correct?

If so when would the primary flag on a work schedule assignment ever be No?