An M.2 drive is simply a term to describe the physical form factor of a drive. M.2 drives can use protocol like NVMe and SATA.  you can get an M.2 drive that uses either SATA or NVMe.

The R7 manual mentions 

• One U.2 drive 
• One M.2 2280 solid-state drive

The U.2 Drive is for NVMe protocol. PCIe NVMe up to 32 Gbps.  The U.2 drive uses M.2 form factor.

The M.2 2280 drive is for SATA protocol.  AHCI.

Thanks that's what I thought or think I did!

However saw this thread with suggested to someone upgrading the M2 drive to get an PCIe drive.  

https://www.dell.com/community/Alienware-Desktops/Aurora-R7-with-M2-SSD/td-p/7346458

And bios screen showing m2 pcie ssd

I have an R7.  The motherboard's M2 interface is M Key (PCiE NVME).  Here is a photo of the NVME controller, and of the motherboard, or you can look at your own PC to confirm there is only 1 single notch on the left side. 

. 

@r72019 is correct.  I think the manual is bit confusing in wording.  

On motherboard there is only one M.2 ssd slot which can accommodate either NVMe or SATA drive.   There is a separate M.2 slot on board for wireless card only.

I would think Dell designed the board that way so user who could not afford one or more NVMe ssd has the option to use the less expensive M.2 sata for the on board M.2 ssd slot.

My post about it here:

https://www.dell.com/community/Alienware-Desktops/Installing-3-x-1TB-M-2-Samsung-PCiE-NVME-Drives-in-Aurora-R7-8/td-p/7474557

Re: saw this thread with suggested to someone upgrading the M2 drive to get an PCIe drive.  

You can install additional M.2 NVMe drive on a PCIe adapter which has gold teeth that inserts into a PCIe slot.  This is separate from the M.2 sata slot built in the motherboard.

https://www.amazon.com/EZDIY-FAB-NVME-Adapter-Express-Support/dp/B07NY25LRD

@r72019 showed a picture of how he did it.

I have an M2 NVME pcie ssd installed on the motherboard.  You can see it in the first photo.  

Okay, I`ll have to have another look at my mobo to see whether the M2 slot is sata or PCIE NVME. However, just by looking at the bios and googling the current SSD drive in that slot, I can see that it is an M+B keyed SSD, and the bios states it is connected via SATA.

So the question is, if I do have an M+B keyed SATA sSD in that M2 slot, is it possible that that same M2 slot  can accommodate an M keyed PCIE NVME SSD? Or are they not interchangeable ie. you need a M keyed slot for a M keyed PCIE NVME SSD, and an M+B keyed slot for an M+B keyed SATA SSD?

it seems the M.2 slot on R7 motherboard is M keyed.  It can accommodate

M.2 SATA SSD, B+M-keyed (can fit in B-keyed and M-keyed sockets)

and 

M.2 NVMe SSD, M-keyed (for PCIe 3.0 x4 lane, can only fit in M-keyed socket)