Originally Posted by
IggySD
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.
The aircraft flying SEA-SFO isn't necessarily the same as that flying SFO-SAN. Therefore it's possible if the SEA-SFO segment is delayed that there could be 2 active flight 431s, hence the need for the designation to avoid confusion.