There are now many types and makes of fly lines on the market. Make sure you understand the use the manufacturer intends of each category of line and its coding, ie, WF is for weight forward, DT is for double tapered, F/S is sinking tip, etc.

The single most important gear set up in fly fishing is that the line # or wt should match the rod # rating you intend using it with.

That is, the ratings are now universal and a #6wt line is manufactured and tested to be most effective with a #6wt rod.