Just because the institution is considered customdian of record does not necessilarly mean they are also a witness. It is the basis of the request and not the request in general.
For example, if a bank receives a subpoena for certain records by the sourt for a divorce case, it will prtovide the records as the custodian of the records. If the attorney wish for the bank to testify to the ligitimacy of the records, then they would be considered a witness.
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The opinion stated here is what it is, My Opinion.