An additional vow usually is an expression of the order or the congregation's charism or particular insertion in the apostolic field of the Church.
The Camillians take a fourth vow of service to the sick, even in danger of death.
[1] After a period of service as a priest, members of the Society of Jesus—referred to as Jesuits—can be allowed to take a fourth vow of obedience to the pope with regard to the missions.
The text of the vow is "...I further promise a special obedience to the sovereign pontiff in regard to the missions, according to the same Apostolic Letters and the Constitutions.
In part VII of the Constitutions, discussing the "distribution of the members in the Vineyard of the Lord" the founding fathers explain the purpose of the fourth vow: "Those who first united to form the Society were from different provinces and realms and did not know into which regions they were to go, whether among the faithful or the unbelievers; and therefore to avoid erring in the path of the Lord, they made that promise or vow in order that His Holiness might distribute them for greater glory to God.