UA is strictly seniority (within the given travel classification). You can check-in on line 24 hours in advance for your own convenience and automatically be put in seniority order in the CSRs' gate computer system so you don't have to stand in any lines at the airport, but the time you checked in on line (or anywhere else) is irrelevant for boarding purposes.At AA (I don't know about UAL), non-rev priority within a given travel classification is first-come-first-served by check-in time. You can check-in from home by computer up to 4 hours prior to scheduled departure time.