An M-Key M.2-SSD can only be installed into a Socket-#3 connector on the motherboard, which will support full PCIex4 lanes, for maximum throughput and performance. An B&M-Key M.2-SSD can be installed into either a Socket-#3 connector or a Socket-#2 connector, The Socket-#2 connector will support a SATA-M.2-SSD or a PCIe-M.2-SSD, operating only PCIex2  See list below:

Key-ID.          Notched Key Pin#               Interface 

B-Key                12 - 19                             SATA or PCIex2

M-Key                59 - 66                            PCIex4 or SATA        

Note: M.2-SSD cards have an Edge Connector on one end, with 75 Pin Positions. With up to 67 pins for electrical connections, at .5mm pitch, that has pins overlap on either side of the PCB-card and have 8 Pin positions dedicated to mechanical/physical keying. The other end of the M.2-SSD card is a semi-circular mounting hole.