When ticketed on United metal, you can stand by on an earlier United metal flight on the same routing on the day of departure. Fare class and seat map are irrelevant, but note that if only Economy Plus seats are available, the flight load is often much greater than the seat map would imply. Passengers without seat assignments would be given seats before anyone would be cleared from the standby list.

If you’re asking about Premium Economy, Business, or Domestic First seats, those would not be available to a standby passenger; however, the upgrade is is cleared before the standby list is. However, if the upgrade list is empty, the economy cabin is full, and there’s a premium cabin with an empty seat, they will not clear you onto that plane.