As said above, rosetail is the actual area that is at the end of the finnage, where as HMs hold the extra rays to create the spread of span to 180 degrees+, the rosetail will consist of another set of rays beyond that area thus giving the effect of a rose petal, the main way to see the rose tail is when the tail is in full spread, when its at the 180 degrees it will give the effect that it really should be spread a lot more to flatten the finnage but b/c it doesnt it gives this wonderful tail type.
the rosetail was created through intense breeding of HMs that held a large amount of final rays and and that have been considered as OHM (Over HM). if im correct in saying
spawning 2 rosetails produce feather tails which is again an extension upon the rosetail rays, and not a good idea to spawn 2 feather tails due to disabilities in the fry from excess weight in the finnage
To keep the rosetail going its advisable to spawn with HMs. the fry should be a certain % of rosetails and HMs.
I would spawn him with a light blue HMBF this should keep the butterfly in most fry but also give some of the fry a deeper metalic tone to the finnage, with possibles of copper, light or dark purples, blues and what an exciting thought of the all these colours in one fish with a rich metalic finish.
now that would be one AWESOME betta in my eyes.