Originally Posted by
hockeyguy
It usually means that United is reusing the flight number, so the add a letter to the call sign to avoid any potential confusion.
Reusing it from what? I looked on flightaware and that flight goes SEA-SFO-SAN every day and is just 431 for both legs. That day (15Jul) it was specifically listed as 431 Tango. I was going to ask the pilot but the beers I had before leaving SFO caught up with me as soon as the fasten seatbelt sign came on for landing so was in a bit of a hurry.