It looks to me (I don't suppose there's a caption or citation for the table that explains?) like it's the number of repeats of the STR motif, plus sometimes a suffix for the number of extra bp for some variants. For example see this about CSF1PO. The entry for allele 8 says the repeat structure is "[AGAT]8" with different amplicon lengths reported for the different sets, but allele 8.3 below that is 3 bp longer in each set. So, 8 repeats of AGAT, plus 3 nt? Or, take allele 9 at the various lengths shown versus 9.1 at 1 bp more than each of those lengths.