Houghy,
In laymens terms it means this:-
The DGM NPB is basically a narrowband or UHC type filter, similar in its bandpass characteristics to the UHC or Narrowband filters made by Astronomiks, Lumicon, Meade, Celestron, Thousand Oaks, Sirius Optics, Baader and Televue. Some of these work a lot better than others, you will note in my previous post I have only recommended 4, some of those listed above I have not tried, but I have tried most of them. All of these filters have a bandpass to varying degrees between about 470nm and 520nm. Some UHC/Narrowband filters as well as having the bandpass between 470 and 520nm also have a higher wavelength bandpass for H-Alpha above about 600nm. Some UHC filters have a clipped bandpass in H-Alpha and some such as the Lumicon UHC and the Orion Ultrablock (both good filters BTW) have no bandpass in H-Alpha. An OIII filter has a narrower bandpass around 500nm only (OIII emission wavelength).
The DGM NPB filter has a narrowband pass between 486 and 501nm and a clipped H-Alpha bandpass above about 620nm. This can be seen in the transmission graph on the DGM website. The Astronomiks UHC which is another filter I like a lot, has similar bandpass characteristics as can be seen in this link on the Bintel website.
http://www.bintel.com.au/AstronomikUHC.html
It has transmission between 480 and 505nm at the lower end and a H-Alpha Bandpass above 640nm.
In other words the DGM filter should work very well on its stated specifications. If Phil Harrington and Mike Palmerti like it wtfru waiting for
Clear Skies
John B