The governing boxing bodys ie WBC, WBA, IBF, WBO etc have all the fighters graded. If a belt is on the line and your graded next in line then you become the mandatory fighter . Most fighters are trying to get up the rankings and by beating opponents ranked higher than you means you go up the rankings and hopefully end up fightjng for a title. Groves has 19 fights unbeaten and became mandatiry fighter. Froch had to fight him or give up his belt. I hope i explained it properly.