A diocesan bishop is not to allow the incardination of a cleric unless: 1/ the necessity or advantage of his own particular church demands it, and without prejudice to the prescripts of the law concerning the decent support of clerics; 2/ he knows by a lawful document that excardination has been granted, and has also obtained from the excardinating bishop, under secrecy if need be, appropriate testimonials concerning the cleric’s life, behavior and studies; 3/ the cleric has declared in writing to the same diocesan bishop that he wishes to be dedicated to the service of the new particular church according to the norm of law.
Source: Code of Canon Law (Vatican.va)