Originally Posted by
ice799
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Does United cap the number of CPUs per flight?
Was this an inventory thing? In other words, maybe the CPU seats came from a fareclass bucket that was used and so United didn't bother waitlisting anyone else?
Maybe I didn't have a high-enough fare class on the return trip (SFO->EWR) ?
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Looks like everyone has covered your main question so I'll try to answer your other questions so it can be more clear.
United will limit the number of CPUs that are cleared ahead of time to save room for people who buy F or last minute changes. Once boarded, they'll fill every F seat until the list is exhausted.
However, regardless of inventory, they should always put you on the upgrade standby list as long as your are eligible.
There are no fare class restrictions to use CPUs or RPUs with few exceptions. GPU must be W or above except on CPU/RPU-eligible flights.. Award tickets are not eligible unless you hold a UA credit card.
Everything above applies to most domestic flights with the exception of those "special" flights" which are not CPU eligible at all shown in the chart.
It is understandable that the upgrade process can be confusing. It isnt 100% transparent and one needs a phD to understand it fully. Unless one fully understands it, it is difficult to complain to a mis-informed UA agent who has absolute confidence.