It is important to use an appropriate booking window when selling memberships that start on "First Usage" to ensure that any reservations that a customer makes with the membership will not fall outside of the actual expiration date of the membership.
"First Usage" memberships do not activate, and thus, do not have an expiration date until a customer is checked into their first class or penalty cancelled with that membership.
When the customer is deciding on the timeframe to make the reservation, they will be able to make reservations on your class schedule within the Booking Window that you've associated with that membership.
For example, let's say that you plan to sell a 1-week non-recurring membership with a booking window of "rolling 14-days." This will allow customers to make reservations for a rolling two-week period in the future. Once the customer takes their first class and is checked in, that membership is now active, and the 1-week expiration date has been set as well. However, with the rolling 14-day booking window the user will technically be able to make reservations beyond when the membership is set to expire (1 week from their first check in). If the customer does this, any reservations made outside of the expiration date for the membership will be grace canceled. Unfortunately, the customer is not warned that these reservations will be canceled.
Therefore, we highly recommend that any memberships that start on the customer's "First Usage" have identical durations and booking windows.