RO6 means the number of hands in the game plus the number of rounds equals 6. That's why a lot of SD tables only have 5 spots. RO5 and RO7 should be obvious. Sometimes you can fool a dealer into giving deeper pen by pulling hands in and out of the game.
In general you don't want to play anything less than RO6 and if you can get RO7, get it, it's awesome.