Comparative fault is the legal rule that allocates responsibility between the parties when more than one person contributed to an injury. The injured party's award is reduced by their percentage of fault.
Missouri uses pure comparative fault: an injured plaintiff can recover even if they were 99% at fault, but the award is reduced by that share. Illinois uses modified comparative fault with a 50% bar: if the plaintiff is more than 50% at fault, they recover nothing.
The fault percentages are decided by the jury (or judge in a bench trial). They affect the math at the end of a case, not the existence of the case.