Federal Reserve Notes are money. By definition of the sovereign = fiat. You own little, if you hold a Federal Reserve Note in your hands, maximum another Federal Reserve Note. That they are not backed by anything else nowadays is another detail.
IOUs like an e-check aka "digital Dollar" are not money or cash. A check is a liability of a bank, either on paper or in bytes redeemable for Federal Reserve Notes. You can't send these notes over an electronic communication network, but the IOUs and then have to walk physically to a branch to get the notes in order to hold cash in your hand. The banks still have to clear these "digital Dollar" payments for cash. For the user it looks like "digital Dollars", but it's just an imitation.
It has something to do with that.
Monetary base:
The monetary base is defined as the sum of currency in circulation and reserve balances (deposits held by banks and other depository institutions in their accounts at the Federal Reserve).
http://www.federalreserve.gov/faqs/money_12845.htm
The federal reserve lends money to the banks:
To minimize the risk that the Federal Reserve will incur losses from lending, borrowers must pledge collateral, such as loans and securities.
http://www.federalreserve.gov/faqs/money_12845.htm
The Federal Reserve can simply create Federal Reserve Notes against collateral. Since the collateral is in the accounts at the Federal Reserve, it is part of the monetary base (pls see above). The Federal Reserve will lend aka create only as much Federal Reserve Notes as there is collateral provided.
Since the Federal Reserve can buy treasuries or mortgages from privates with created Federal Reserve Notes, known as QE, it increases the monetary base by the same amount as the Federal Reserve Notes in circulation.