§1. Whenever it is necessary or opportune in order to carry out the pastoral care of a parish fittingly, one or more parochial vicars can be associated with the pastor. As co-workers with the pastor and sharers in his solicitude, they are to offer service in the pastoral ministry by common counsel and effort with the pastor and under his authority.
§2. A parochial vicar can be assigned either to assist in exercising the entire pastoral ministry for the whole parish, a determined part of the parish, or a certain group of the Christian faithful of the parish, or even to assist in fulfilling a specific ministry in different parishes together.
Source: Code of Canon Law (Vatican.va)