Spectral coefficients computed from overlapping MDCT coefficients. MP3 and AAC apply the MDCT only after first splitting the signal into frequency bands with windowed filterbanks. The majority of audio compression comes from allocating bits between different frequency bands based on psycho acoustic information. The concept of window switching to reduce pre-echo effects is patented in US5285498. Critical Bands Group coefficients in critical bands.
Critical bands are a way to group frequency bands which better mimics the response of the human ear.
The concept is old, but there may be patents on the use of critical bands for audio compression. Associated to each critical band is a scale factor. The larger the scale factor, the more bits allocated to this critical bands. This is all that is meant by Quantization. Choose scale factors so quantization distortion in each critical band is less than the masking computed by the psycho-acoustic model.