File format specification is Variant Call Format (VCF).
BCF is a binary representation of VCF files (section 6 from format spec doc above) :
BCF2 is a binary, compressed equivalent of VCF that can be indexed
with tabix and can be efficiently decoded from disk or streams.
and
The “text” field contains the standard VCF header lines in text
format, from ##fileformat=VCFv4.3 to #CHROM ... inclusive, terminated
by a NUL character.
thank you!