- CC,CT,CA,TT,TA,AA}.
-\item {\tt GL} is an extension to and is backward compatible with the
- {\tt GL} genotype field in {\tt VCFv4.0}.
+ CC,CT,TT,CA,TA,AA} (NB: the ordering is different from the one in the original
+ BCF proposal).
+\item Predefined {\tt FORMAT} fields can be missing from VCF headers, but custom {\tt FORMAT} fields
+ are required to be explicitly defined in the headers.
+\item A {\tt FORMAT} field with its name starting with `{\tt \_}' is specific to BCF only.
+ It gives an alternative binary representation of the corresponding VCF field, in case
+ the default representation is unable to keep the genotype information,
+ for example, when the ploidy is not 2 or there are more than 8 alleles.