There are two waitlists - the "priority" waitlist (PA), and the "standard" waitlist (PB). PB is not touched util PA is empty. When you are waitlisted, you really don't know which list you're on unless you specifically ask (and there's no way to find out online).

In general, a SWU or CR-1 will put you on PA, while a mileage upgrade will put you on PB (though some 1Ks report they end up on PA).