From my memory back in my AA gate agent days 1983 - 1995 ...
HK = hold confirmed
PN = pending. Other airline shows availability but has yet to respond as HK
UC = unable to confirm .. other airline said no go to HK
WL = waitlisted.. ticketed, HK not avail, on list to change to HK if/when able
NS = no show, all downline segments auto cancelled
UG = upgraded due operational reasons, no bonus miles
DG = downgraded due operational reasons, refund due
NN = need.. pretty please, can I have it ? Responds back as PN, UC or HK
SS = force sell.. responds as HK (agent must have the authority in their record & only works for the agt's airline)
DS = desires.. pax may show up w/ticket but is not HK
MM = mealist.. employee nonrev, create a PNR and plan for meal & weight/balance but don't remove seat from inventory
Many others. Some may be AA specific. Some are "request" types, other are "status" types. Some are "interim" types, ie: there's another code for "unable to confirm" than UC and it means the passenger has been advised ... don't remember it cuz usually you cancel the requested segments and try something else.
Trivia...
To request a First Class seat on American flight #9001 from New york Kennedy to Los Angeles on June 31, the entry is (on Sabre .. and it's similar thruout the airline industry):
0AA9001F31JUNJFKLAXNN1
(it's funny how today, 10 years after leaving, I still abbreviate the word "need" as "NN")
Newer programs just give the agent a mask to fill in. Faster training, but slower time processing the passenger cuz you have to "tab" to each data field. For those agents who know (were given the training), the "native" program and its cryptic (but one-step) entry was actually much quicker.
And if the above "NN" request came back as HK (and for simplicity say that you already have a ticket on United and it's transferable to AA), I could finish your reservation (it's actually called a PNR - Passenger Name Record) with:
-JONES/ROBERT&9817-555-1234&7T-&E
Sorry I went on-and-on .. it was fun pulling this stuff outta my head