Originally Posted by robthree

An ID90 can be purchased for either coach or a premium cabin. (90% off the cheapest unrestricted 'Y' fare -or- 90% off the cheapest unrestricted 'A' or 'J' fare.) If you buy the premium pass and there are no premium seats you can be seated in coach, but will get no refund of the fare difference. If you buy a pass in coach there will be no upgrade. You can buy both and apply for a refund of the unused ones.

Economy plus is a coach seat. If they are available, as a non-rev, you will likely be assigned one. When they clear you from the standby list you will be assigned boarding group 3. Although they may not clear you until all revenue pax have boarded already, making the boarding priority moot.