See
https://farpointastro.com/product-ca...ialty-filters/
Lumicon was taken over by Farpoint but I refer you to them because I am familiar with their products and they were one of the first if not the first to market these many years ago.
Nebula filters are contrast enhancing filters available in different bandwidths. Broad, narrow and line filters are available.
The Lumicon Deep Sky filter is a broadband filter that gives a mild boost to contrast. The UHC is a narrow band filter that gives a significant boost to contrast and contains both the O-III and H-Beta lines. The O-III and H-Beta are line filters centred on specific parts of the spectrum.
Many nebula emit in O-III, especially planetary nebula so the O-III filter is ideal for these whilst some objects are stronger in H-Beta so the H-Beta filter is better for these objects. The UHC is my recommendation as a first filter followed by an O-III and then H-Beta.
There are a few brands like Lumicon, Astronomik and Thousand Oaks that I have used and recommend and many others that I have not tried.
As for the Rosette nebula it is a very large object so you need a 2" low power eyepiece with a 2" filter and reasonably short focal length telescope to see it.
See also
http://www.prairieastronomyclub.org/...p-sky-objects/