Though it is more common to see BTC in most casual settings, XBT is used under certain circumstances in the larger global financial system. There is a universal standard named 'ISO 4217', used by banks and financial institutions, which dictates the format for currency codes. According to ISO 4217, non-national or international currencies like Bitcoin should begin with the letter 'X'.
In practical terms, there is no intrinsic difference between XBT and BTC. They both represent the same thing -- Bitcoin. The choice between using XBT or BTC is merely a matter of preference.
In some cryptocurrency exchanges, you might encounter the acronym XBT as an identifier for Bitcoin. This is especially true for exchanges that operate within traditional institutional finance platforms or work closely with banks. However, in more mainstream and consumer-oriented platforms, you're more likely to encounter BTC.