Rileyc123

I recently bought a new heater/ ac box because I'm planning on putting ac in and was looking at it and I'm wondering...what does the the big opening on the left side go into to? And where does it break off into separate vents? I have the original heater control box with no ac and from what I've seen on the internet it does not fit with an ac box...is this true and if so what can I do? Please help