Its actually R inventory for Y > J sticker upgrades on domestic 3-class flights. And , no, I have never seen either R or C on Carlson. Only one I know of that shows R and C is ExpertFlyer.com.

The reason you do see A on Carlson is because it is also a revenue fare (YUPs for the next few weeks, corporate discounts, eVIPS, and RTW fares) in addition to mileage upgrades.