Revenue buckets
J - Full Fare Business
C - Discount Business
D - Discount Business
Z - Deep Discount Business
P - Deep Discount Business
O - Full Fare PremiumPlus (Premium Economy)
A - Discount PremiumPlus (Premium Economy)
R - Deep Discount PremiumPlus (Premium Economy)
Y - Full Fare Coach
B - Full Fare Coach
M - Discount Coach
E - Discount Coach
U - Discount Coach
H - Discount Coach
Q - Discount Coach
V - Discount Coach
W - Deep Discount Coach
S - Deep Discount Coach
T - Deep Discount Coach
L - Deep Discount Coach
K - Deep Discount Coach
G - Deep Discount Coach
N - Basic Economy

Saver Award Inventory
IN - Elite (Plat/1K/GS) Saver Business or 2-cabin First Award
I - Non-Elite and Low Elite Saver Business or 2-cabin First Award
XN - Elite Saver Coach Award. Also available to Chase Gateway, Explorer, Quest, and United Club Visa cardholders as an undocumented benefit
X - Non-Elite Saver Coach Award

"Everyday" Award Inventory
JN - Elite/CC Everyday Business or 2-cabin First Award (may be open to all MP members )
ZN - Non-Elite Everyday Business or 2-cabin First Award (tracks to the Z bucket) (not clear if still used for award travel). Also used for buy-up upgrade offered at check-in. May be a factor in Skip-the-Waitliist availability
ON - Everyday PremiumPlus (Premium Economy) Award
YN - Elite/CC Everyday Coach Award (may be open to all MP members)
HN - Non-Elite Everyday Coach Award (tracks to the H bucket) (not clear this is still used)

Upgrade Inventory
PN - Y/B/M 1K Instant Upgrades (CPU eligible routes only); All GS sponsored upgrades Y->J (CPU, PlusPoints, miles)
PZ - Upgrades to Polaris Business, United Business, or United First (excepting GS sponsored upgrades and 1K Instant Upgrades)
RN - Upgrades to Premium Plus

Points earning for all upgrades is based on the originally purchased fare. Also see below.

Upgrades Note that upgraded flights will post to your MileagePlus statement as Z(X) where the second letter (X) is the original booking class. The parenthetical letter defines the earning rate for the flight as points are earned based on the original booking class.


How to read inventory numbers:
Why does J8C5D3Z0P0 mean?
The zero inventory is easy to explain, UA is presently unwilling to sell tickets using inventory in that fare class.
Does J8C5D3 mean there are 16 seats for sale? No -- there are just 8 available seats.
Inventory in a fare class is not additive but the lower fare classes are subsets of the higher fare classes
If UA sells / upgrades 1 seat in the premium fare class, the Inventory will like change to J7C4D2.
The J inventory is the total number of seats for sale in business/first. While C,D,Z, P ... are the number of seats UA is willing to sell in those lower fare classes
Going back to J8C5D3Z0P0, if you want 4 seats on a single PNR, you will have to purchase C fare, as there isn't enough inventory for D fare for everyone on the PNR and everyone on a common PNR must have the same fare class / fare basis.

In some cases, if a lower cabin is oversold, UA will then block, remove from inventory in the higher cabin to protect against the oversale.
Note the largest number for a UA fare class in public displays is 9 (some airlines limit to just 4), this prevents revealing too much information to the competitors or the public.

Award and upgrade inventory are also subsets on their respective cabin inventory

Management of inventory and maximizing revenue is the role of Revenue Management (RM), a highly secretive, proprietary operation of the airlines -- So inventory changes (within the physical limitations of the aircraft) will be dynamic and can change without notice as determine by RM

Archive thread: Decoding the alphabet soup - fare buckets for UA {Archive}