Auto tray switching is on for jobs that don't specify a tray. So if you tell the driver to print to tray 2 and it has 11*17 it will empty the tray and stop.
If you tell the driver to print on 11*17 it will print to a tray with that in it, and when it empties it, it will move on to the next tray with 11*17 in it. The order in which the trays go is the priority (1 lowest is top priority) of the tray. 2 is whether the device should allow switching with that tray (You disable switching for things like letterhead or some other tray you don't want people to use by accident)
EWS > Properties > General Setup > Paper Management > Tray Content & Settings
