Relative Clauses of Purpose
You learned that Latin often shows the purpose of an action with ut followed by a subjunctive. You can also form a purpose clause with a form of quīfollowed by a subjunctive. The difference is that in the ut construction, you are showing the purpose behind the action of the main verb. With quī, you are showing the purpose of the pronoun's antecedent.
Caesar nuntium Romam mī sit ut auxilium peteret. (Caesar sent a messenger to Rome to ask for help. [why he sent the messenger])
Caesar nuntium Roman mī sit quī auxilium peteret. (Caesar sent a messenger to Rome to ask for help. [what the messenger was supposed to do])
There is a related construction called a relative clause of characteristic. It is used to make vague, general comments about people or things. For example, Sunt quī ova nō n ē ssent (“There are people who don't eat eggs,” or “Some people don't eat eggs”). By using the subjunctive, the statement becomes less a statement of fact as it is a hypothesis or opinion.