The Gen 5-6 is the 5 Series UV system rated for a flow rate of 6 Gallons Per Minute (GPM) and takes the 40040014 Lamp which is 470 mm long. The Gen 5-10 is still the 5 Series UV System but it is rated for 10 Gallons Per Minute (GPM). It also takes the much longer 40040015 Lamp which is 820 mm long.