To add on to what others have said, for most bargaining units and for rank and file staff here's my theory on the order in which you should take your time off (excluding sick):

1. PDD (expires each fiscal year)
2. PLP (required to be taken before all of the following options)
3. VPLP (caps at 240 hours for nearly all BUs)
4. Vacation or Annual Leave (caps at 640 hours)
5. ITO (can be taken hourly, no cap, never expires, can't be cashed out)
6. Personal Holidays (must be taken as a full day/unit, no cap, never expires, can be cashed out)
7. Holiday Credit (happens when a holiday falls on a Saturday, can be taken hourly, no cap, never expires, can be cashed out)