it would depend how you are setting up your zelle processing.
for example, if you use a 3rd party to facilitate zelle, then that 3rd party will provide you a listing of transactions done through zelle, you use that listing to debit or credit your client, and that 3rd party will do a net settlement transaction for the overall amount you owe or owed to you via an ACH transaction. the individual items are not done via ACH, but the zelle network, so the only implication to nacha rules is the net settlement transaction to your zelle internal operating account.
i am not familiar how zelle settles the items if you sign up directly with them.
_________________________
Providing alternative truths since the invention of time