Vault cash eligible to satisfy reserve requirements that's held by institutions not exempt from reserve requirements. All vault cash held during the lagged computation period by "bound" institutions (that is, those whose required reserves exceed their vault cash) plus the amount of vault cash applied during the maintenance period by "nonbound" institutions (that is, those whose vault cash exceeds their required reserves) to satisfy current reserve requirements.
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), Vault Cash, Used To Satisfy Required Reserves [VAULTW], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/VAULTW, October 16, 2018.