Instead of receiving a withdrawal benefit that is immediately available on resignation or dismissal, you may choose to leave your entitlement in PSS and receive the benefit later.
If you choose to defer (i.e. leave your benefit in the scheme), it can be paid later as a lump sum.
Why have a deferred option?
The deferred benefit is generally of greater value than a benefit immediately available as it will include the employer-financed benefits accrued during your service.