Reserve balance requirements series is the portion of the reserve requirements not satisfied by vault cash. The series is calculated as Required reserves (https://fred.stlouisfed.org/series/REQRESNSW) less Vault cash used to satisfy required reserves (https://fred.stlouisfed.org/series/VAULTW).
Effective February 2, 1984, reserve computation and maintenance periods have been changed from weekly to bi-weekly. Series with data prior to February 2, 1984 have different values reported from one week to the next. After February 2, 1984, the value repeats for 2 consecutive weeks.
Board of Governors of the Federal Reserve System (US), Reserve Balances Required; Reserve Balance Requirements [RESBALREQW], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/RESBALREQW, March 21, 2018.