A rel=canonical link tells search engines which URL is authoritative, preventing duplicate-content dilution across parameterised or syndicated copies of a page.
Impact is importance multiplied by the confidence tier’s weight — SPECIFICATION 1.0, RESEARCH 0.7, EMERGING 0.3, EXPERIMENTAL 0.1. Two rules backed by the same class of evidence therefore always carry the same weight, which is what makes the score reproducible rather than hand-tuned.