A reference level equal to "Full Scale." Used in specifying A/D and D/A audio data converters. Full scale refers to the maximum voltage level possible before "digital clipping," or digital overload of the data converter

BFS means "decibels full scale". It is an abbreviation for decibel amplitude levels in digital systems which have a maximum available level (like PCM encoding). 0 dBFS is assigned to the maximum possible level.