The mixing phase consists of multiple interventions such as : track cleaning (reducing the singer's breath, the monitoring feedback, etc), sibilances and plosives (noise gate, de-esser), audio & MIDI quantification, equalization (subtractive or additive, M/S), compression (full-band, multiband, parallel or sidechain), the reamping, effects (delay, reverb, saturation, harmonic generation, etc), track mixing itself (volume and pan), parameters automation, mix-down creation by analog or digital summing ...
Some of these operations will be essential, others will only be justified based on the intrinsic quality of the tracks.
Track rendering may also require further audio restoration processing : hiss reduction, click removal, crackles, hum or distorsion of any kind would falled within this framework.
The role of mixing is to make harmonious the listening of the mixed tracks that make a song. Its goal is not to reach a high volume or calibrated volume for most listening systems. These interventions consist in a later phase, named the mastering.