Multiband compressors typically offer 3 exclusive bands but some are known to be customizable with bands of up to 5, 6 or even 7. As frequencies have their own relative characteristic, behaviour, and desired response based on genre, processing these frequencies in their own exclusive band brings a further degree of control of the overall frequency spectrum. Bass frequencies can be compressed harder for more presence while mid and treble frequencies can simultaneously use less compression to allow for more dynamic movement all within the same device.
Multiband compression is not only a mastering tool but can also be used for vocals, buss processing, gating, drum kits or some extreme uses can help sound design artists achieve unpredictable artefacts.
As a typical rule of thumb, compression during mastering should never achieve more than 1 or 2dB of gain reduction, however, bass frequencies should utilize the higher end of that range while upper frequencies benefit from less compression allowing the sound to breathe while creating strength in the low end.
Before compressing, ensure the crossover frequencies do not intersect and important sounds and that they are set in such a way where they deal with a similar set of frequencies relative to one anothers' inherent characteristics. For example, the lower band can capture the sub, bass, kick and other fundamentals of the rhythm section. The middle band can capture first order harmonics, vocal fundamentals and tonal instruments. The upper band can capture the grittiness, clarity and brightness of sounds.