You have more than one event or campaign running simultaneously and need to determine the source of funds coming into your bank account for accounting purposes.
One of the requirements of having multiple events all managed through a single administration area is that all events must use the same payment processor and therefore, all event funds will be deposited into the same bank account.
For Authorize.net and PayPal clients, the Event Name will be included in the Description field of the payment receipt. For other processors, the Sub-hostname will be listed. These fields are both located at Events -> Settings.