id: numeric tag id, the corresponding string is bcf_hdr_t::idBCF_DT_ID[$id].key
n: number of values per-sample; size: number of bytes per-sample; type: one of BCF_BT_* types
same as vptr and vptr_* in bcf_info_t below
size: number of bytes per-sample; type: one of BCF_BT_* types
type: one of BCF_BT_* types
FORMAT field data (§1.4.2 Genotype fields)