What is the concept of excommunicating a specific individual (takfir al-mu'ayyan), and what is the difference between it and excommunicating an unspecified individual (takfir ghayr al-mu'ayyan)?

Excommunicating a Muslim is among the major sins that are forbidden unless there is clear evidence, for we do not excommunicate anyone except those whom Allah and His Messenger have excommunicated. There is a difference between excommunicating a specific individual and declaring an act to be disbelief, because declaring an act to be disbelief does not necessarily mean that the doer is a disbeliever if there is an excuse. For example, sitting on a Mushaf (copy of the Qur’an): the act itself is disbelief, but the doer's situation is considered. If he knew its sacredness, then he is a disbeliever; otherwise, he is not.