The correspondent account and its use in tax enforcement in the context of offshore banks is described as:
A correspondent account is a bank deposit account maintained by one bank for another bank. Financial transactions involving U.S. dollars flow through U.S. banks. Therefore, foreign banks that do business in U.S. dollars, but have no office in the U.S., obtain a correspondent account at a U.S. bank in order to engage in such transactions. These transactions leave a trail in the U.S. that the IRS can access through the records of the correspondent bank accounts. These correspondent bank accounts have records of money deposited, money paid out through checks and money moved through the correspondent account by wire transfers. All of this information the IRS can obtain through a John Doe summons issued to the U.S. bank holding the correspondent account.As best I understand it, through the correspondent bank, the offshore bank without any other U.S. presence can service U.S. customers for some of their needs. But, the U.S. bank establishing the correspondent relationship -- in this case, Wells Fargo -- has a U.S. presence which means that it is subject to U.S. process, including a John Doe Summons which can be used to ferret out information of the offshore bank's customers using the relationship. The use of such correspondent bank relationships to service U.S. customers needs is the weakness in offshore banks' claims of impregnability to U.S. tax and law enforcement because they have no U.S. presence. A similar JDS was issued to UBS's U.S. branch for records of its correspodent relationship with Wegelin & Co. See prior coverage on the Wegelin related JDS. IRS Issues John Doe Summons to UBS (All Over Again) (1/28/13; updated 2/2/13), here.