What do the numbers in the preferred IUPAC name for ascorbic acid mean?
The numbering of (R)-5-[(S)-1,2-dihydroxyethyl]-3,4-dihydroxyfuran-2(5&#8203;H)-one has four parts:

1) Numbering of the furan ring



2) Numbering of the ethyl side chain



3) Stereodescriptors R and S



4) Italicized element symbols

The italic element symbol H denotes indicated or added hydrogen.