Sorry, won't go hunting for the interesting-sounding discussion jc94 refers to, but a few data points from asking concierges:
1) On a midweek YUL-FCO, was told I was one of 2 "revenue J" pax - I was on a P fare.
2) On YYZ-SCL, have been waitlisted in paid-J and told no upgrades were granted, all revenue J. As a SE100K 1MM, I was the only person who cleared the paid J standby waitlist at the gate.
3) On short transborder, told there were no revenue J, all were upgrades
4) I foolishly assumed YVR-YOW would be no or very few revenue J, but told by concierges up front was "nearly full" of revenue J - largely connecting from Asia on revenue J tickets.