PRE is used to help schedulers know about outside commitments staff may have. The preferred off day is not related to PTO, it is only a request to not be scheduled.  That time is unpaid and assumes they will be scheduled for their full hours around approved preferred days off. 

There is a 14 day restriction so that schedulers have two weeks in advance to make adjustments around such requests. However, you can accomplish the same thing by booking off the requested shift to code ADJ which means a schedule adjustment to remove a shift.

PTO does not have a time restriction and be requested and applied in the current period or in the future.