I'm unclear about this as well.

For example I know that for conventional (non direct-vent) appliances you need 1 sq/in net free per 2K BTU (Input) if air is provided from the outside via horizontal ducts.

But sites like this one

Combustion Air

seem to be stating that a gas appliance installed in a confined space requires the same connection to a interior unconfined space irrespective of whether it is also supplied with exterior air.

Hopefully someone thoroughly familiar with the NFGC will be along to set us straight.