The Stablecoin Supply Ratio (SSR) is the ratio between Bitcoin supply and the supply of stablecoins, denominated in BTC.
When the SSR is low, the current stablecoin supply has more "buying power" to purchase BTC. It serves as a proxy for the supply/demand mechanics between BTC and USD.
SSR is calculated by dividing the total market capitalization of BTC by the total market cap of all (known and tracked) stablecoins in circulation.
Rafael Schultze-Kraft and Renato Shirakashi - Stablecoins' Buying Power over Bitcoin