Here you find the description of the BTL format as PDF file.
This documentation 10.6 is dated 10.05.2022:
btl_v106.pdf
Obsolete Versions:*
This documentation 10.5 is dated 13.12.2012:
btl_v105.pdf
This documentation 10.4 is dated 23.06.2011:
btl_v104.pdf
This documentation 10.3 is dated 26.11.2009: btl_v103.pdf
This documentation 10.2 is dated 14.07.2009: btl_v102.pdf
This documentation 10.1 is dated 13.07.2009: btl_v101.pdf
This documentation 10.0 is dated 12.01.2008: btl_v10.pdf
*These versions are not updated any more, they are only available for the sake of completeness. Please use one of the newer versions of the documentation for actual work.