The SRF is a ceiling. If overnight repo threatens to spike because reserves are scarce, dealers can always get cash from the Fed at the SRF rate against government collateral, so the market rate has little reason to trade far above it.
It was created after the September 2019 repo spike, when overnight rates briefly jumped to 10% because reserves had been drained too far by earlier balance-sheet-runoff.
Example: the SRF rate is 5.50%. GC repo starts printing 5.48% into quarter-end. Dealers tap the facility for $30bn overnight and the rate settles back to 5.35%.
Related: repo, overnight-reverse-repo-facility, iorb, bank-reserves, discount-window