BASSenc_MP3 is an extension to BASSenc that allows BASS channels to be MP3 encoded without needing an external encoder (eg. LAME). It can be downloaded from the BASS website:

The BASSENC_MP3.CHM file should be copied into the same directory as this file (BASSENC.CHM), to be able to conveniently access the BASSenc_MP3 documentation from within this BASSenc documentation.