Rule 1565 If more than one subpoena duces tecum is served upon the custodian of records or other qualified witness and the personal attendance of the custodian …

When multiple parties in a case all send subpoenas to the same records keeper, asking that person to show up in court, there needs to be a rule about who "owns" that witness. The law says the witness belongs to whichever party sent their subpoena first.

This matters because the party who called a witness gets to question them first. If you were the first to subpoena the records keeper, you get to ask your questions before the other parties do.