Q.Distinguish between the $R/S$ (CIP) system and the $D/L$ (relative) system for describing the configuration at a stereocentre. Which system is based purely on atomic-number priorities, and which is based on comparison with a reference compound?

The $R/S$ (CIP) system assigns a configuration by ranking the four substituents at a stereocentre purely by atomic number (working outward on ties), then reading the sense of rotation (clockwise = R, counterclockwise = S) with the lowest-priority group pointing away from the viewer. It requires no external reference compound and can, in principle, be applied unambiguously to any stereocentre in any molecule -- it is therefore called an absolute system. The $D/L$ system instead assigns a configuration by comparing a Fischer projection of the compound in question to that of a fixed, historically chosen reference compound, glyceraldehyde: a compound whose configuration at the relevant carbon matches D-glyceraldehyde's spatial arrangement is labelled D, and one matching L-glyceraldehyde is labelled L. Because this labelling depends entirely on that external comparison rather th …

Keep the two systems' basis distinct: R/S from priority rules applied to the molecule itself; D/L from spatia …

Assuming D and R (or L and S) always correspond to each other -- they happen to coincide for glyceraldehyde itself but the two systems are i …