XBT is the alternative ticker symbol for Bitcoin, often used interchangeably with BTC.
XBT, also known as digital gold is the acronym used to follow the specifics of the ISO 4217 standard guidelines of other non-national or supranational currencies which include gold (XAU), silver (XAG), platinum(XPT), and more.
The X indicates that the currencies have no particular country border. Thus, Bitcoin is much more a gold and silver currency than the dollar or other country-tied currencies.
BTC and XBT can be used interchangeably but XBT is more used in a professional environment such as derivatives trading.