Signifies, with its superscript, the bonding number, i.e. the sum of the number of skeletal bonds to an atom and the number of hydrogen atoms associated with it in a parent hydride, real or hypothetical. Its usage in names of compounds is generally restricted to non-standard valence states.
PAC, 1984, 56, 769. 'Treatment of variable valence in organic nomenclature (lambda convention) (Recommendations 1983)' on page 774 (https://doi.org/10.1351/pac198456060769)