The Rectus (R)- and Sinister (S)- prefixes are different from the preceding ones in that the labels R and S characterize the absolute configuration of a specific stereocenter, not a whole molecule.
If there is a pair of enantiomers, each with one stereocenter, then one enantiomer is R and the other is S, and likewise one enantiomer is levorotary and the other is dextrorotary.