Signifies, with its superscript, the @B00703@, i.e. the sum of the number of skeletal bonds to an atom and the number of hydrogen atoms associated with it in a @P04405@, real or hypothetical. Its usage in names of compounds is generally restricted to non-standard @V06588@ states.
PAC, 1984, 56, 769. (Treatment of variable valence in organic nomenclature (lambda convention) (Recommendations 1983)) on page 774 [Terms] [Paper]
See also:
Blue Book, p. 465 [Terms] [Book]
Red Book, 3rd ed., p. 245 [Terms] [Book]