A stereoheterotopic face of a trigonal atom is designated
Re if the ligands of the trigonal atom appear in a clockwise sense in order of CIP priority when viewed from that side of the face. The opposite arrangement is termed
Si.
See also: enantiotopic, prochirality
Source:
PAC, 1996, 68, 2193. 'Basic terminology of stereochemistry (IUPAC Recommendations 1996)' on page 2216 (https://doi.org/10.1351/pac199668122193)