Ultimaker 2 Jet Nozzles The Olsson block brass Jet nozzles have a special tip angle and shoulder width, they also have a special internal geometry with an angle less steep down to the nozzle hole. A special treatment applied inside also makes them slip more. You can print 3°C to 5°C lower because of this treatment, and is also useful for oozing and drips.

With these special nozzles, you can print faster, the plastic flows much more easier than in classic nozzles. You can also clean them efficiently with cold pulls (or atomic pulls). The geometry of the inside makes it easier to get rid of plastic so changing colours or filament type is done faster and safer. Use the 0.25mm brass nozzle to print small detailed prints, it's an expert nozzle (RS 110-1982)
The 0.4mm (RS 110-1983) is the standard size for a lot of 3D printers
If you print in wood filaments, this brass nozzle of 0.8mm is perfect for the job (RS 110-1985)