-\item A {\tt FORMAT} field with its name starting with `{\tt \_}' gives an alternative
- binary representation of the corresponding VCF field. The alternative representation
- is used when the default representation is unable to keep the genotype information,
- for example, when the ploidy is over 2 or there are more than 8 alleles.
+\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.