This excerpt taken from the C 10-Q filed Nov 5, 2007.
Commercial and similar letter of credit
A commercial letter of credit is an instrument by which Citigroup substitutes its credit for that of a customer to enable the customers to finance the purchase of goods or to incur other commitments. Citigroup issues a letter on behalf of its client to a supplier and agrees to pay them upon presentation of documentary evidence that the supplier has performed in accordance with the terms of the letter of credit. When drawn, the customer then is required to reimburse Citigroup.