Full Scale (FS) means full scale. An analog Full Scale value is so high that the corresponding electronic circuit, such as an AD converter, is fully modulated and corresponds to a digital value of loud ones: 1111 1111.
The 1s value is the reference value for 0 Decibel Full Scale (dBFS). All other dBFS values are given in relation to this value. The maximum value of 0 dBFS must not be exceeded, otherwise the modules will distort the signals.
The smallest dBFS value is the value with all zeros: 0000 0000. Depending on the quantization and its resolution, the lowest dBFS value will be 0000 0000 0000 at 12-bit resolution; at 16, 20 or 24 bits it would be correspondingly more zeros. Since the numerical specification is too confusing, the full-scale ratio is specified in dBFS. If a signal level reaches 50% of the maximum value, the dBFS value is -6 dB. Digital values with 16 bit resolution correspond to a dBFS value of -96 dB, 20 bit to such a value of -120 dB and 24 bit to -144 dB.