Plastic compartment beneath battery tray?

I’m following vacuum hoses in my '90 Sovereign and wondering what the plastic compartment beneath the battery tray is? There is a vacuum hose going to it through what appears to be a check valve. I don’t have a vacuum pump to see if this compartment might be the source of a vacuum leak I’m hunting and thought I’d seal off the hose to see if it makes a difference. Before doing so, I’d like to know what the compartment is so that disconnecting the vacuum hose doesn’t do anything harmful. Can’t seem to find it in any of the Classic Parts diagrams. Does anyone know its purpose?

You mean this ? -

Bryan_N thank you!!! I never considered that was part of the HVAC and therefore never looked at those diagrams. My smoke machine leak detector never indicated a leak there but not being able to see a large portion of the compartment, I couldn’t be certain whether there was a problem or not. Again, many thanks!