I can speak to how it works for card processing and ACH is most likely similar. To participate in payment processing in the banking network, you have to have a Merchant ID that is tied to a bank account. The processor or gateway is holding a suspense/escrow account on your behalf throughout the day and when a batch of transactions settles, it will resolve the balance difference with your bank account. The amount of payments allowed into or out of your escrow account is set by the processors based on your company's financial health and a risk analysis since if you just debited say $10 million from the escrow account and you only had $5 in your account, the processor would need to collect that debt from you, and they do not have a guarantee that they'll be able to do so. This is how it works for debit cards and bank accounts since $ amounts are real. It's slightly different for credit cards because the $ amount is in a way fictional, so they don't do the escrow holding and just temporarily "allocate" part of the credit limit (this is called an authorization) and when it is settled this is "captured", which enqueues the authorization for future processing. A few days later it will process and be included in a lump sum of funding into your merchant account. This reply is my personal understanding and meant for educational reasons and doesn't represent opinions or viewpoints of any company, and should not be considered advice of any kind and it may be inaccurate.