Mojha
This is one of the biggest can of worms in fly fishing!
I will say that the numbers on the rod are only a guide. You will have to make up your own mind as to what is the best line on the rod for you. The guy who rates the rod doesn't know if you are:
- a 90 year old first time caster or
- a discus thrower who has been fly fishing since boyhood or
- a river fisherman who never has more than 20 feet of line out the tip ring or
- a sea trout fisherman who is casting weighted flies
You get the picture!
It's his best guess.
So I have a 6/7 weight rod that I use with lines from 6 to 9. "Why a 9" you may ask? Well some of my fishing is very short casting on a small salmon river with heavy flies - but only short distances and a heavy line handles the weighted flies better than the lighter lines. For the distances I am casting on that river it is the perfect combination.
Some rods are intended for particular roles. The TCR 5 weight can carry 80 feet of very long belly 5 weight line and then shoot 30 feet or more. It's an out and out distance casting rod rated 5. Some people would use that rod with 6, 7 or even 8 weight lines for normal fishing.
So coming back to what I first said - it's a guide and you make up your own mind as to what's best for you and the purpose you want to put the rod to.
One of the greatest of all fly fishermen was a guy called Arthur Cove. He used to fish long distance nymphs. He used a 5 weight double taper line on a 7/8 weight rod as that allowed him to lift 20 yards of line then re- cast to a cruising fish.