Without X-Frame-Options (or a CSP frame-ancestors directive) the page can be embedded in an attacker's iframe and used for clickjacking.
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.