Bringing together community feedback and extensive testing by E3D, they have redesigned their nozzles from the ground up.

The new internal geometry of v6 Nozzles is optimised to reduce back-pressure, this ensures smooth,easy flow of filament whilst improving the effectiveness of retractions. The bottom flat surface of the nozzle which flattens down the layer is custom to each nozzle size, for small nozzles a small nozzle tip gives you high precision. On the larger nozzles, a nozzle tip assists in the smooth printing of larger layers. 

All nozzles in the series are now individually identifiable with markings on the flats of the hex head making them even more convenient to work with. See the table below for more information on what these markings mean:

Compatibility

• v6 Nozzles are backwards compatible, so can be used to upgrade your v5 HotEnd.
• If you are using a v6 HeatBreak it is recommended that you use a v6 Nozzle.

- See more at: http://e3d-online.com/E3D-v6/Extra-Nozzles/v6-Extra-Nozzle-1.75mmx0.25mm#sthash.ok8n0ogu.dpuf