The amount of securities held by Federal Reserve Banks. This quantity is the cumulative result of permanent open market operations: outright purchases or sales of securities, conducted by the Federal Reserve. Section 14 of the Federal Reserve Act defines the securities that the Federal Reserve is authorized to buy and sell.
Board of Governors of the Federal Reserve System (US), Reserve Bank Credit: Securities Held Outright [WSECOUT], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/WSECOUT, July 19, 2019.
Total factors supplying reserve funds are the sum of "Reserve Bank credit," "gold stock," the "special drawing right certificate account," and "Treasury currency outstanding.
Board of Governors of the Federal Reserve System (US), Factors Supplying Reserve Balances: Total Factors Supplying Reserve Funds [WTFSRFA], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/WTFSRFA, July 19, 2019.