The fully ordained Buddhist monk is bound by a procedure of exceptional exactness and, at the same time, by no expectation of permanence, and the two facts sit together in a way that surprises those who assume monastic vows are for life.
Procedure and the grave offences
Ordination requires a quorum of existing monks, ten in a settled region and five in a border region, performed within a properly determined boundary with a formal motion and three announcements, and an ordination with a defective quorum or boundary is invalid. The monk is bound by the Patimokkha, 227 rules in the Theravada recension, recited fortnightly. Four of them, sexual intercourse, theft, killing a human being, and falsely claiming spiritual attainments, are parajika, and breaking one expels the monk permanently and automatically, with no judgment required and no re-ordination in that lifetime.
Temporary by design
Ordination is not for life by design. In Thailand, Myanmar, Laos and Cambodia temporary ordination is expected of most men, sometimes for a rains retreat, sometimes for a few weeks, and disrobing carries no stigma at all. This differs sharply from the Christian understanding of an indelible character, and it means the same person may be a monk more than once. Monks own almost nothing, eat nothing after midday, and depend entirely on lay donation, which is the material basis of the relationship between sangha and laity.