Quote currency
The quote currency is the second half of a trading pair, the currency the price is expressed in: in BTCUSDT it is USDT, in BTCUSDC it is USDC.
Pairs with different quote currencies are separate markets with separate order books, so BTCUSDT, BTCUSDC and BTCUSD show slightly different prices at the same moment. When you compare two charts, check that they quote the same currency.
Learn more: where to find a good price chart.