Rev St st = reverse stockinette stitch
When you are knitting one row and purling the next, in general, the side with the 'V's is considered to be the right side.
In reverse stockinette, the side with the bumps (the purl side) is considered to be the right side.
Which side is facing you?
As with standard stockinette when knitting flat, if the V's are facing you, you are on the knit side and should knit the row. If the bumps are facing you, you are on the purl side, so purl the row. When knitting in the round, purl all stitches to create reverse stockinette.