>>Any idea about the payback days? I'm thinking if I have these, I could have used them instead of my no credit APD, no?<<

Possibly if there was adequate reserve coverage. If you can hold a schedule, you can also use them to drop trips, where they will likely be worth more than dropping a reserve day.

There are other options detailed in 23.S.16. of the contract. Basically there are varying ways to convert Payback Days into vacation. Many guys think of this as less desirable though since vacation pays less per day than dropping a trip or even a day of reserve.

There is also some information in the Scheduling reference Handbook around page 87-- here's an excerpt:

Q: Can I use payback days in a regular line month to drop a trip? How will I be paid?
A: Yes, by mutual agreement with Crew Scheduling. You may enter your request in PCS
under “Leave Requests” by selecting “Payback Days.” Your pay will depend on the number
of payback days available for use. If the number of days available are sufficient to cover
the entire rotation, that number of days will be used and you will be paid for the entire
rotation. If the number of days is not sufficient to cover the entire rotation, all available
payback days will be placed on the days of the rotation, beginning with the first of such
days, and you will be paid the value of those days only. You will forfeit any pay or
guarantee for the remainder of the rotation. Note that credit resulting from application of
duty rigs is always tied to the last duty period of the rotation.
Q: Can I use a payback day to drop a reserve on call day? How will I be paid?
A: Yes, by mutual agreement with Crew Scheduling. You may enter your request in PCS
under “Leave Requests” by selecting “Payback Days.” Your reserve guarantee will not be
affected by the days awarded. You will be credited a reserve pro rata share for each on
call day dropped.
Q: Can I have payback days added to my current year vacation? How will I be paid?
A: Yes, by mutual agreement with Vacation Planning. Make this request by contacting the
Vacation Planning Office. Your vacation bank will be increased by 3:15 per day added, and
the days will subsequently be paid as any other vacation days are paid.
Q: What happens to my payback days if they are not used in the current year?
A: If the days have not been used prior to January 1, they will automatically be converted
to supplemental (SUPP) vacation days to be used in the subsequent vacation year. If you
wish to use the days in January, February, or March, you may enter a request for a
personal drop, and then contact a Crew Scheduling supervisor to have the days of the
personal drop converted to payback days, if desired.