Looking at Fedwire® Funds Service - Monthly Statistics it appears that these transactions are between banks but I'm trying to confirm is whether the stuff they are exchanging is classified as reserves. That is, FR liabilities shown in "Deposits with F.R. Banks, other than reserve balances" in H.4.1?
When sending a payment order to the Fedwire Funds Service, a Fedwire participant authorizes its Federal Reserve Bank to debit its master account for the amount of the transfer. If the payment order is accepted, the Federal Reserve Bank holding the master account of the Fedwire participant that is to receive the transfer will credit the same amount to that master account. Each payment order sent through the Fedwire Funds Service is processed individually upon receipt through an encrypted electronic network.
The above came from the Fedwire product sheet.
So yes, it is a transfer of a balance held at a Federal Reserve bank.