You're right about learning... I went and Googled "air systems: Air Make-up Systems" deciding to try it literally. Never heard of this before, although I did have an inspired image of aeroplane cabin air systems when I saw your words, which sent me looking..
I got some stuff about closed loop air conditioning systems that use a small amount of external air to make up the right balance of oxygen, or temperature, or humidity. It could get complex, but from what I read, the idea is to make it simpler and cheaper than standard air conditioning. What I didn't find out, was whether the 'circuit' was the air flow circuit, of a gadget with electronic parts designed to regulate that air flow and mix.