Is MAC prep and prime a primer or setting spray?

I'm curious, is MAC prep and prime classified as a primer or a setting spray? I've heard differing opinions and would like to get a clear understanding of its purpose and function. Is it intended to be used as a base for makeup to ensure a smooth, even application, or as a finishing product to help set makeup in place and prevent it from fading throughout the day? I'd appreciate any clarification on this matter.