About
Monofilament Lines are fishing lines made from a single fiber. They are cheap to produce and come in a variety of different diameters, so flexibly fit a range of different situations.
― In-game description
This is the Monofilament 10 kg fishing line in COTW The Angler.
Its test weight is what the scoring system charges you for. A catch is worth 1300 points per kilogram of fish and costs 300 points per kilogram of line rating, so the same fish scores higher on lighter line. Land one heavier than the line is rated for and that charge comes back multiplied, which is where the biggest scores come from.