An 'ad clerum' (from the Latin meaning 'To / All Clergy') is a formal letter or announcement primarily addressed to ordained ministers, from the Bishop(s) of the Diocese.
Issues given this formality are usually serious and the equivalent of a parliamentary 'three line whip' - i.e. there are serious consequences for those with the Bishop's Licence who do not take heed.
Below is a list of all the Ad Clerums issued by Bishop Michael and the Area Bishops.