The M.2 socket can hold either a NVMe M.2 SSD or a SATA M.2 SSD.
If you use a SATA M.2 SSD, you could put a similarly-sized 2.5" SATA SSD in the drive bay and use RAID0 or RAID1 across them. You could also put a HDD in the drive bay and use all or part of the M.2 SSD as a high-speed cache (this is Intel SRT and is akin to Optane Memory).
If you use a M.2 NVMe SSD, there is no way to use Intel RST unless the NUC has two M.2 sockets. Only the NUC6i7KY and the new NUC8i7HN and NUC8i7HV NUCs have two M.2 sockets.
Hope this covers it,