When you sign up for an authorize.net account if you already have a merchant bank account you are asked for the merchant bank information on the application. This is how they link authorize.net to your bank.
If you have applied for both an Authorize.net account and Merchant account through Authorize.net, you will receive notices from both Authorize.net and the merchant bank that you are able to process transactions. Transactions will only process correctly after you receive your second notice stating that your merchant bank is active.
Here is an example of the second notice from a merchant bank. In this example, the email is from CyberSource.