The Federal Open Market Committee (FOMC) instructs the Reserve Bank of New York as to how it should use the SOMA to support monetary policy.
Following the global financial crisis of 2007-2008 leading to the Great Recession, the FOMC "increased the size and adjusted the composition of the SOMA portfolio in efforts to promote the Committee’s mandate to foster maximum employment and price stability".
[3] Interest on the portfolio provides virtually all of the Fed's income, but the central bank buys and sells securities purely to implement U.S. monetary policy and not for profit.
Participation in the foreign portfolio is determined first by allocating each Reserve Bank a share in proportion to its year-end capital and surplus.
The COVID-19 pandemic interrupted that process, and the account increased again to a peak of $8.5 trillion in April 2022, then proceeded to unwind again at a faster pace than before.