Below you will find a brief explanation along with a helpful diagram.
Mitigation is a term employed to design the means and measures in place that reduce the negative effects of a DDoS attack.
Mitigation consists of filtering illegitimate traffic and hoovering it up with the VAC, while letting legitimate packets pass.
The VAC consists of multiple devices, each with a specific function to block one or more types of attack (DDoS, Flood, etc.).
Depending on the attack, one or more defense strategies may be put in place on each VAC device.