This excerpt taken from the JEF 10-K filed Mar 1, 2006.
In connection with both our trading and brokerage activities, we borrow securities to cover short sales and to complete transactions in which customers have failed to deliver securities by the required settlement date, and lend securities to other brokers and dealers for similar purposes. In addition, we have an active matched book business whereby we borrow securities from one party and lend them to another party. When we borrow securities, we provide cash to the lender as collateral, which is reflected in our financial statements as receivable from brokers and dealers. We earn interest revenues on this cash collateral. Similarly, when we lend securities to another party, that party provides cash to us as collateral, which is reflected in our financial statements as payable to brokers and dealers. We incur interest expense on the cash collateral received from the party borrowing the securities. A substantial portion of our interest revenues and interest expenses results from our matched book activities. The initial collateral advanced or received approximates or is greater than, the fair value of the securities borrowed or loaned. We monitor the fair value of the securities borrowed and loaned on a daily basis and request additional collateral or return excess collateral, as appropriate.