And what is its importance?

It stands for Attitude Control System. It manages what the satelite recognises and how it reacts to it. In this case, the Russians reprogrammed it so their fighter planes wouldn't be recognised.

If you guys look at the cinematic at the start of the mission "Wolverines!", NORAD struggles to identify why "boggies" are appearing out of nowhere. And if you look to the left, a title shows(Attack Characterization System) which if you think of it's acronym the title stands for ACS. A NORAD member thinks one of the ACS modules are malfunctioning. So ACS means Attack Characterization System. I literally made this account for this exact reason.

#LtColMuradru maybe correct as well since Attitude Control System (AOCS in real life) is also an aerospace thing but Attack Characterization System seems a bit more likely not only because it has a similar description with it's real life counterpart and makes more sense but also it is actually presented on-screen.