ISDAfix rates at the center of a U.S. price-manipulation probe, while often relegated to the fine print of regulatory filings, help determine everything from borrowing costs on bonds that finance skyscrapers to interest on annuities.

Companies seeking to protect against soaring interest rates in the $379 trillion swaps market rely on them to mark the value of trades. Banks use the rates in setting coupons paid for $550 billion of bonds tied to commercial real estate. Fluctuations help determine the performance of structured notes bought by wealthy individuals and the amounts some states pay on pension annuities.

After fining Barclays Plc, UBS AG and Royal Bank of Scotland Group Plc more than $2.5 billion over the past year for rigging the London interbank offered rate, a global benchmark for $300 trillion of securities, regulators are now expanding their examination into measures that, while obscure, hold as much sway in the workings of the world's financial system.