The intermediate lines are supposed to have a similar density as water or just slightly more. They are sinking lines with sink rates 1-2 i.p.s. (inches per second). They will sink slowly when left still but not when they are retrieved (or so goes the gospel). We cast them out and count to say 3 and retrieve. Next cast we count to six etc. Thus, we can check various depths. The lake above has an Arctic Char subspecies that eat water snails on the bottom. To catch those, we hover just above the bottom (lava). “They take just before you snag the bottom” goes the saying, and it is true.
Intermediate lines are conventionally in darker colors than floaters. They are an excellent choice from just below the surface to maybe 10′ down. It is easy to roll them to the surface before commencing the forward cast.
There are excellent clear intermediate lines now on the market. If we fish with clear intermediate lines we can get by with a short leader, say 2′. Then the fly gets pulled down where we want it to go. I like to use clear intermediate lines when fishing the surf of the Gulf, or when I am chasing False Albacore.