The stated purpose for such enclosure is to prevent distraction from prayer and the religious life. Depending upon the reason and the length of time, the proper authority (usually the superior gets approval from the local bishop or the Holy See) can allow enclosed men or women to leave the enclosure "for a grave cause and with the consent of the superior". In the Catholic Church, this matter is regulated within the framework of canon law and of the particular statutes approved by the Holy See.
Enclosed religious orders of men include monks following the Rule of Saint Benedict, such as the branches of the Order of Saint Benedict itself, the Cistercian Order, and the Trappist Order, but also monks of the Carthusian Order, Hieronymite monks, and some branches of Carmelite friars, along with members of the Monastic Family of Bethlehem, while enclosed religious orders of women include Canonesses Regular, nuns belonging to the Benedictine, Cistercian, Trappist and the Carthusian Orders, along with nuns of the second order of each of the mendicant orders, including: the Poor Clares, the Colettine Poor Clares, the Capuchin Poor Clares, the Dominicans, Carmelites, Servites, Augustinians, Minims, together with the Conceptionist nuns, the Visitandine nuns, Ursulines nuns (as distinct from "federated Ursulines" of the Company of St. Ursula) and women members of the Monastic Family of Bethlehem.
The English word monk most properly refers to men in monastic life, while the term friar more properly refers to mendicants active in the broader world (like Franciscans, Dominicans and Augustinians), though not all monasteries require strict enclosure. Benedictine monks, for instance, have often staffed parishes and been allowed to leave monastery confines.
Although the English word nun is often used to describe all Christian women who have joined religious institutes, strictly speaking, women are referred to as nuns only when they live in papal enclosure; otherwise, they are religious sisters. The distinctions between the Christian terms monk, nun, friar, Brother, and Sister are sometimes easily blurred because some orders (such as the Dominicans or Augustinians) include nuns who are enclosed, who are usually grouped as the Second Order of that movement, and Religious Sisters who work in the broader world, who form a part of its Third Order.
In the Roman Rite of the Catholic Church, once a man or woman has made solemn, perpetual religious vows, the release from these monastic vows has to be approved by the ecclesiastical authorities. Normally there is a transitional period, called exclaustration, in which the person looks to establish a new life and determine if this is what he or she is truly called to do. This usually lasts up to six years under the current Code of Canon Law. After this period the appropriate authority, generally the Holy See, determines that the wish to leave this life is valid and grants the former monk or nun release from their vows.