for burning double layer DVD's. For a long time DVD's had only a single layer of material that could be burned to by a laser. In order to increase the amount of data that can be stored on a DVD, the double layer DVD's were developed.
In some cases each layer is burned by a different wavelegth laser. The blank dual layer DVD's cost considerably more than the standard single layer jobs.
As with the single layer there are two opposing c]specifications supported by various industry groups. Thus with Double layer there are still the minus and the plus competiton, [most burners now burn either type. The next step is Blu-ray and of course the competing DVD[something, forgot the name] which can have more layers, use shorter wavelength lasers and can thus put more data on a single disk.
At this time a blu-ray burner costs about $1000.00.
There are absolutely no sign that the industry will ever settle on a single standard in DVD technology. Like Betamax and VHS.