BASSenc_FLAC is an extension to BASSenc that allows BASS channels to be FLAC (Free Lossless Audio Codec) encoded without needing an external encoder. It can be downloaded from the BASS website:

The BASSENC_FLAC.CHM file should be copied into the same directory as this file (BASSENC.CHM), to be able to conveniently access the BASSenc_FLAC documentation from within this BASSenc documentation.