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.
Transformed and rewritten from PDF version (entry
http://www.iupac.org/goldbook/R05308.pdf)
by: Miloslav Nic, Jiri Jirat, Bedrich Kosata, ICT Prague, Czech Republic