Their FAQ is definitely unclear, but, as mentioned in a previous HN discussion, a 5V/3A is going to work fine for most workloads. They limit downstream USB current to 600mA. If you need the full 1.2A limit, you can buy their power supply, another (not common) one, or a powered USB hub.
5V/5A will also give you 5W extra power on the device for their high performance mode.
But again, it’s not necessary and most uses of a Pi will do fine with 5V/3A.
