The R, S and r, s notations are used to provide absolute stereochemistry according to the Cahn-Ingold-Prelog (CIP) priority rules (This rule originates with the Cahn-Ingold-Prelog 'Rule 5').
The notation (R) and (S) are used to provide Standard tetrahedral stereochemistry. However, (r) and (s) are used to provide Tetrahedral stereochemistry determined by other stereochemical centers.
Say the lower case r and s refer to stereocenters that depend upon other stereocenters (if the compound has more than one chiral atom - stereocenters)
More specifically, when two substituents on an atom differ only in their absolute configuration (R or S), if the relative priorities of these substituents need to be established, R takes priority over S. When this happens, the descriptor of the stereocenter is a lowercase letter (r or s) instead of the uppercase letter normally used.
ChemDraw updates stereochemistry indicators as you change your drawing.
To include stereochemistry indicators in your drawing:
1. Select the structure.
2. Right-click and select Object Settings from the context menu.
3. From the Drawing tab, under Atom Indicators and Bond Indicators, select Show Stereochemistry.
The stereo-centers are marked, as shown in this example of cholesteral:
See the ChemDraw User Guide in section "Stereochemistry Indicators" for more information.