The SB field of HaplotypeCaller output is not described terribly well as far as I can find.
##FORMAT=<ID=SB,Number=4,Type=Integer,Description="Per-sample component statistics which comprise the Fisher's Exact Test to detect strand bias.">
What exactly happens when there are multiple alternate alleles?
For example:
scaffold_1 2535 . T C,TTC,<NON_REF> 611.83 . DP=15;MLEAC=1,1,0;MLEAF=0.500,0.500,0.00;MQ=60.00;MQ0=0 GT:AD:DP:GQ:PL:SB 1/2:0,5,10,0:15:99:630,397,379,210,0,180,607,394,210,604:0,0,12,3
It doesn't seem to be particularly informative in this case (a case which is rather common for our data).
If it isn't already part of the possible annotations...
Perhaps the most sensible approach would be to output field with num-fwd, num-rev for each allele (rev, alt1, alt2, ...). SDP for "strand-depth" might be a reasonable name.