''Fighting skill.'' One of the basic ways of acquiring a high combat rating is, of course, to get a high fighting skill. Your fighting skill gives different combat rating depending on whether you are fighting an NPC creature, or you're fighting in PvP combat.
''Normal fighting (fighting subskill).'' "Max combat rating bonus for normal fighting was raised to 2.5" -Release notes July 12, 2011
''Kingdom bonus.'' An Elevation- and Chaos-only feature, there are two steps to this kingdom bonus. If your kingdom has less than 20% of the server online, you receive an extra 1 combat rating. If your kingdom constitutes less than 10% of the server population, you gain another point of combat rating, for a total of 2 extra.