United, along with most other airlines, separate their available seats into three “buckets”:

• Seats that you can buy with money
• Seats that are available for redemption using miles
• Seats that are available for upgrade from a lower class

There are only a limited number of seats available on any flight, and the airlines constantly try to adjust their pricing as well as availability in the various buckets to maximize their return. This mechanism is very fluid, and takes into account factors such as traffic load, competitor’s fare promotions, weather conditions, economic factors, proximity to the date of flight, number of seats already sold, fares that the seats were sold for, and many other factors. The results is that there is no simple way to predict with 100% certainty the buckets that would be available at any given time.

Many airlines make it very difficult to find out the inventory available in the various buckets, however, United provides its customers a tool called “Expert mode” to check for upgrade inventory.

To set your user status to expert mode in MileagePlus, complete the following steps:
1. Log into UAL.com
2. Go to your Mileage Summary page
3. On the left hand side, under your name and account balance, click the link “My profile”
4. Click “Edit” in the first section on the left “My travel preferences”
5. On the fourth section “Display preferences”, the first field is “Availability format”, select “an expert”. Save the change

Now, whenever you want to see the upgrade inventory of a flight:
1. Go to the “Planning and Booking” menu at the top, do not click, but look for “Booking with Miles”
2. Select the radio button at the top “Standard award”
3. Enter the origination and destination you want, and the date, leave the Service class as Economy.

When the available flights show up, move the mouse cursor to “>Flight info”, and keep the cursor there
A pop-up will come up, displaying the route, equipment, and most importantly, the Availability.
The format will be something like this: F5, P0, A0, Y8, B8, E7, M8, U4, H1, Q0, V0, W0, T0, S0, K1, L0, NF0, XF0, NC0, XC0, G0, NY0, XY0.

The first part of the list shows the revenue seat availability, starting with the most expensive class (F), to the least expensive (L).
- F, P, and A are all First class seats, F is full fare, A is discounted F
- Y, B, E, M, U, H, Q, V, W, T, S, K, and L should be all Economy class

Here is the key to deciphering the rest:
NF – Upgrade to First class
XF – Award First class seats, using miles
NC – Upgrade to Business class
XC – Award Business class seats, using miles
G – not sure
NY – Award Economy seats, using miles, extra inventory reserved for 1K.
XY – Award Economy seats, using mles

The number behind each of the classes is the inventory available for each. Maximum of 9. For upgrades to First, you want to look for NF.

The best use of the above technique is BEFORE you purchase your ticket. If you have flexibility in the dates and time of your travel, then by searching for the flight with NF availability, you could then book those flights, and know that you will be able to immediately confirm your upgrade without having to be placed on a waitlist (where you may or MAY not ever be upgrades into First). Of course, before you actually purchase the ticket, you should call United to confirm that there is indeed NF availability before the purchase.