Just checked in for a flight tomorrow on a CRJ700, right on 24 hours out and as expected seats 7C and 7D showed as available, and I was able to select 7D.
Checking the seat map on the app a few minutes later showed 7C taken, so I presumed someone had managed to grab it too, but when I look on the website it shows 7C as being "Perm Block" :
However if I run through the check-in process again, I show in 7D, and 7C shows as available for selection.
Doing a dummy booking whilst not logged in gives me the option to select (for payment) any of the other E+ seats, but shows 7C as taken.
Very much looks like these seats are being reserved based on status level - although I have no way of telling what the minimum status is.