It is an M.2 2230 slot for the AC-3168 slot.
Other wireless cards have been validated as documented below:
No, it cannot be used for storage. This M.2 socket is of type A/E and storage devices require a M.2 socket of type B/M. In theory it has two PCIe lanes and I suppose that, at least physically, it could host a slower x2 drive, but I do not believe that the BIOS is ready to support the NVMe steerage needed for this.