The table lays out the keys in common use today—there are others, mostly placeholders to be called into service as newer buses and interfaces are introduced.
The four – or five-digit numbers paired with each slot are the physical dimensions of each card; the first two digits specify the width in milli-meters and the second two or three digits specify the length. Typically a Wi-Fi module is 16mm wide and 30mm long, or 1630.
All current keys can give cards access to two PCI Express lanes, but otherwise interface compatibility is all over the place—so far, it’s been pretty easy to guess what kind of peripheral you’re dealing with based on the key it uses. Wi-Fi and W WAN cards tend to use keys A and/or E, since they only need the PCI Express or USB 2.0 buses and only need 30mm in length to fit all their key components. SATA SSDs and SSDs that use two PCI Express lanes tend to use keys B and M to maximize compatibility, since both connectors can deliver both SATA III and two PCIe lanes. The very fastest SSDs tend to be M-keyed since it’s the only one that delivers four PCIe lanes.