There are 2 Account Number fields defined in this segment, in ACT01, and in ACT06. Both are labeled as assigned. Can you confirm these values are assigned by the Payer?
Technical Report Type 3 for 834 Benefit Enrollment and Maintenance (005010X220A1) does not explicitly state by whom the TPA/Broker Account Numbers (loop 1100C elements ACT01 and ACT06) are assigned.
While it is reasonable to assume these account numbers are assigned by the payer, senders are advised to confirm with the receiver