Normally on rods you'd get a recommended line class, like 7-15kg's or 15-30lb's. Thats generally the suggested line class that'll suit the rod. In most cases, i've found that the lower of the rating to be the one most applicable, like in the 7-15kg example, 7kg would be the most suitable.
Reels also have recommended line ratings. Take a 2500 sized fixed spool reel. It'll be rated to carry say 120-150' of 10lb line. With the advent of braid, which is considerabily thinner, you could get maybe the same amount of line on the reel, but in 30lb+ breaing strain. Reels are generally designed to cope with about 30% of the breaking strain of the maximum strength of line recommended. So, a 2500 sized reel, would be able to cope with 3lb drag (30% of 10lb), but not with the 30%of 30lb braid. Thats why manufacturers now give maximum drag rating on their reels. Especially on the bigger class reels, where anglers are now using 40-130lb braid on fixed spools.
Reels also have recommended line ratings. Take a 2500 sized fixed spool reel. It'll be rated to carry say 120-150' of 10lb line. With the advent of braid, which is considerabily thinner, you could get maybe the same amount of line on the reel, but in 30lb+ breaing strain. Reels are generally designed to cope with about 30% of the breaking strain of the maximum strength of line recommended. So, a 2500 sized reel, would be able to cope with 3lb drag (30% of 10lb), but not with the 30%of 30lb braid. Thats why manufacturers now give maximum drag rating on their reels. Especially on the bigger class reels, where anglers are now using 40-130lb braid on fixed spools.