§1. The judge or the president of a collegiate tribunal can designate an auditor, selected either from the judges of the tribunal or from persons the bishop approves for this function, to instruct the case.
§2. The bishop can approve for the function of auditor clerics or lay persons outstanding for their good character, prudence, and doctrine.
§3. It is for the auditor, according to the mandate of the judge, only to collect the proofs and hand those collected over to the judge. Unless the mandate of the judge prevents it, however, the auditor can in the meantime decide what proofs are to be collected and in what manner if a question may arise about this while the auditor exercises his or her function.
Source: Code of Canon Law (Vatican.va)