These fields are defined in the header of the VCF, and are copied below for convenience:
##FORMAT=<ID=GT,Number=1,Type=String,Description="Genotype"> ##FORMAT=<ID=GP,Number=G,Type=Float,Description="Genotype Probabilities"> ##FORMAT=<ID=DS,Number=1,Type=Float,Description="Estimated Alternate Allele Dosage"> ##FORMAT=<ID=RC,Number=1,Type=Integer,Description="Count of reads with REF allele"> ##FORMAT=<ID=AC,Number=1,Type=Integer,Description="Count of reads with ALT allele">