Cyrille,
B50 is simply a model designation. I believe it is the only Dacron material currently offered by Brownell. You build the string with a strand count (such as 10, 12, 14, 16, etc.) appropriate for the bow's weight, and B50 can be used on a bow of any poundage (just as the low-stretch materials can).
The individual strand's "break strength" rating come into play to obtain the most efficient and safe strand count for a particular bow's weight. The strand's diameter also comes into play when you want to accomplish an adequate strand count in combination with a particular serving's diameter.
Cruise Lancaster's site and look at the specs for all the different string materials (the print catalog will give it to you in a single glance over a couple of pages). They also print a generic recommended strand count for each material, although this is variable based upon bow weight and personal taste.
You can hot-rod any string with differing strand count as long as you at least have a safe minimum that will suit the bow's weight (as well as ensuring that the loops are thick enough to avoid damage to the bow's tips). Some like the uber-skinny builds with padded loops, others like ample girth throughout the entire string.
It's vast ... and very cool to study and experiment with.
Hope this helps.