What Is The Meaning Of “sn” In The Lipid Name?

From “Nomenclature of Lipids”, IUPAC-IUB Commission on Biochemical Nomenclature (CBN) (https://www.qmul.ac.uk/sbcs/iupac/lipid/):

Lip-1.13. Stereospecific Numbering. In order to designate the configuration of glycerol derivatives, the carbon atoms of glycerol are numbered stereospecifically. The carbon atom that appears on top in that Fischer projection that shows a vertical carbon chain with the hydroxyl group at carbon-2 to the left is designated as C-1. To differentiate such numbering from conventional numbering conveying no steric information, the prefix ‘sn’ (for stereospecifically numbered) is used.

These Rules are as close as possible to the published version prepared by the Working Group on Lipid Nomenclature: H. Hirschmann (U.S.A.), P. Karlson (Federal Republic of Germany; convenor), W. Stoffel (Federal Republic of Germany), F. Snyder (U.S.A.), S. Veibel (Denmark), F. Vögtle (Federal Republic of Germany) and the Working Group on Glycolipid Nomenclature: S.Basu (U.S.A.), R. O. Brady (U.S.A.), R. M. Burton (U.S.A.), R. Caputto (Argentina), S. Gatt (Israel), S. I. Hakomori (U.S.A.), M. Philippart (U.S.A.), L. Svennerholm (Sweden), D. Shapiro (Israel), C. C. Sweeley (U.S.A.), H. Wiegandt (Federal Republic of Germany; convenor) [see Biochem. J., 1978, 171, 21-35; Chem. Phys. Lipids, 1978, 21, 159-173; Eur. J. Biochem., 1977, 79, 11-21; Hoppe-Seyler’s Z. Physiol. Chem., 1977, 358, 617-631; J. Lipid Res., 1978, 19, 114-128; Lipids, 1977, 12, 455-468; Mol. Cell. Biochem., 1977, 17, 157-171; Biochemical Nomenclature and Related Documents, 2nd edition, Portland Press, 1992, pages 180-190. Copyright IUPAC and IUBMB; reproduced with the permission of IUPAC and IUBMB].