Cases are reserved either In most cases, one's Ordinary (usually one's bishop) has the ability to lift ecclesiastical censure, although certain sins are reserved to the Apostolic See, including host desecration, assaulting the Pope, breaking the seal of the confessional, and consecrating another bishop without permission from the Pope.
In a case of urgent necessity, when it is not possible to have recourse to the proper superior, an ordinary priest may absolve a penitent, directly from unreserved sins and indirectly from episcopal reserved cases, but the penitent must afterwards apply to the person having power to absolve from the reservation.
It is certain, however, that a bishop has authority to declare that ignorance of a reservation does not prevent its incurrence in his diocese.
Ignorance of a censure (excommunication, interdict, removal from the exercise of certain ministries, or for clerics- deacons, priests, or Bishops, suspension from certain faculties), or of a reservation (to the Pope or the Bishop or Ordinary) attached to a particular mortal sin, are both distinct from ignorance of its status as a mortal sin.
Persons who have reached the age of reason (about 6 or 7 years), have previously received the Sacrament of Reconciliation, and who possess the mental and other faculties needed to commit serious sins, and who committed the act (of omission or commission) with full knowledge and intent and were aware of its seriousness, may not receive Holy Communion- or any other prohibited Sacraments, if they are aware of the censure attached- before being properly absolved.