Concrete subflooring is acceptable as regular flooring, as long as it's in a moisture-proof environment.

What is the difference between flooring and subflooring?

The subfloor is the bottom most structure of the floor. It provides a flat and stable surface and supports the actual floor. Underlayment is different from the subfloor in that it rests between the subfloor and the actual floor.

Does subfloor have to be tongue and groove?

For subflooring, you use tongue and groove plywood. The tongue and groove are only along the 8 foot edges. On the four foot edges, your seams will fall on joist tops, so no need for tongue and groove.