$BTC generally refers to its quotation against another currency, whether fiat (like USD, EUR) or crypto (like ETH, USDT). It is one of the most liquid and relevant pairs in global markets. Its behavior reflects not only the strength or weakness of Bitcoin but also the market's perception of the other currency in the pair. In pairs like BTC/USDT, it is used to measure the volatility of BTC against a stablecoin, while in BTC/ETH, the competition between two leading cryptos is observed. It is key for technical analysis and trading strategy.