Found this while trying to figure out.
M.2 SSD Keying
Currently an M.2 SSD has either one of three key types: B, M or B+M, while a socket can only have one key. As the key positions of B and M are slightly different, the M. SSD can only be installed one way. m.2-connectors A 'B' keying (pins 12-19) gives PCI Express SSDs up to 2x lanes of bandwidth, while a 'M' keying (pins 59-66) provides PCI Express SSDs with up to 4x lanes of bandwidth. Even with 2x lanes of bandwidth a 'B' keyed M.2 SSD still gives 10Gbit/s performance, whereas the 4x lanes on 'M' gives up to 20Gbit/s. M.2 SSDs with B+M keying maximize compatibility in both slots, and will operate with 2x lanes of bandwidth.
ref: https://goo.gl/UEL3g5
