https://doi.org/10.1351/goldbook.R05260
  1. The
    configuration
    of any
    stereogenic
    (asymmetric) centre
    with respect to any other stereogenic centre contained within the same molecular entity. Unlike
    absolute configuration
    , relative configuration is reflection-invariant. Relative configuration, distinguishing
    diastereoisomers
    , may be denoted by the configurational descriptors R*, R* (or l) and R*, S* (or u) meaning, respectively, that the two centres have identical or opposite configurations. For molecules with more than two
    asymmetric
    centres the prefix
    rel
    - may be used in front of the name of one
    enantiomer
    where R and S have been used. If any centres have known
    absolute configuration
    then only R* and S* can be used for the relative configuration.
    See also:
    α (alpha), β (beta) (Defs. 1 and 3)
  2. Two different molecules Xabcd and Xabce, may be said to have the same relative configurations if e takes the position of d in the tetrahedral arrangement of
    ligands
    around X (i.e. the pyramidal fragments Xabc are superposable). By the same token the
    enantiomer
    of Xabce may be said to have the opposite relative configuration to Xabcd. The terms may be applied to
    chiral
    molecular entities with central atoms other than carbon but are limited to cases where the two related molecules differ in a single ligand.
Both definitions can be generalized to include
stereogenic units
other than
asymmetric
centres.
Source:
PAC, 1996, 68, 2193. (Basic terminology of stereochemistry (IUPAC Recommendations 1996)) on page 2217 [Terms] [Paper]