This used to be a quota of two seats per flight for Platinum members and as it was an unpublished benefit, it was usually very easy to find availability. You could also call in advance to check if there was space available and hold the space while booking the ticket.

I am not sure when it changed but the benefit is no longer quite as useful. As far as I understand it, some revenue management software now decides and you can only check if there is availability after you have booked the ticket. There appears to be no way of predicting the outcome; I got refused on a flight which was J7 C7 I7 O7 (and eventually upgraded at the airport) but recently had success for an upgrade on a flight which was J4 C0 I0 O0 during a major sports event (and the flight was eventually overbooked in J).

In theory I suppose it should be possible to buy a Flex Plus ticket to check and then cancel it but I am unsure if the software considers initial booking fare bucket when opening upgrade space.