Single Adjustable shocks mean that when you adjust the shock looser or stiffer you are adjusting both the rebound and the compression together, the shock will be loose or stiff both ways.
Double Adjustable shocks mean that you are adjusting the rebound and the compression independently. You could adjust the shock to have a loose rebound for great weight transfer and a stiffer compression for a more controlled landing. Double adjustable shocks give you the most shock adjustment control of any other choice.
"R" Series front shocks are adjustable just like a single adjustable front shock but the compression adjustment range is stiffer. This allows you to set the shock to a 90/10 type setting at the drag strip and go back to a stock shock type setting when driving on the street. They are designed for front drag racing applications to maximize weight transfer.