Fly lines dimension numbers pertain to weight- the greater the number, the heavier the line. Larger lines are relied on with larger flies and also fish, and also lighter lines are utilized for smaller sized flies as well as fish. There are some handy graphes available to aid figure out which line dimension they need to be utilizing.
Usually, the larger the weight of line the better the line speed so it is a smart idea to take that into account. It will frequently have even more influence as it hits the water too. There are times when a much more fragile hand is desired to not scare the grains from the fish with the line slapping the water.