“M” and “B” are different connector types for M.2 slots. A “B-Key” enables SATA or PCIe NVMe™ SSDs using up to 2 PCIe lanes, and an “M-Key” enables using up to 4 PCIe lanes.
Please note that Samsung NVMe™ SSDs use the “M-Key” (to leverage PCIe x4 for maximum performance) and cannot be plugged into an M.2 slot with a “B-Key”.