Doug, you're idea of D classifications are a little off. The revenue passengers do not have such a classification at all. Rather, these apply only to non-revenue travelers. The first of the non-rev travelers to board are those fly on "A" passes. Most A passes are for business travel, but personal emergency travel (for serious illness, hospitalization, or death) are A-9. These passes are available to a long list of relatives and apply when the emergency involves someone from another long list of relatives.
After the A passes, we get into the 3 main classifications for stand-bys, the D passes.
D-1 passes are available to the same people as D-2 passes, but offer a higher boarding priority (i.e. D-1s board before D-2s). These passes carry the same service charge as D-2s, but are available on only one round trip per year per eligible traveller.
D-2 passes are the standard passes used by employees, there spouses, and their dependent children. They offer unlimited travel throughout the American route system.
D-3 passes are the passes that employees can give to anybody else they want, including family members not eligible for D-2 (such as grandkids, nephews, aunts, etc.). These passes can also be given to friends. Employees are given a total of 24 o/w D-3s per year. D-3s have a lower boarding priority than D-2s.
Within each classification, there are also "through passengers" who are in the computer as "D-1T", "D2-T", or "D-3T". Passengers who are in the middle of their trip are listed as through and have boarding priority over passengers who are just beginning their trips. For example, when I fly SAN-ORD-IND for school, I have boarding priority in Chicago over other D-2s who are starting their trip at O'Hare.
When all of this is put together, this is what the personal travel boarding priority looks like for American:
A-9 -> D-1T -> D-1 -> D-2T -> D-2 -> D-3T -> D-3
Among the travellers in any given classification, boarding passes are issued on a first-come, first-served basis based on initial check-in time.