A Q5 is a Cree LED that is more efficient than others, meaning it puts out more lumens per watt.
The higher the letter and number, the more efficient, and as a result, brighter the LED is.
Most lights with Cree's are P4 bin, although some lights can be special ordered witha Q2, but only special modifications can be made to put a Q5 in. No lights are being manufactured with these emitters.