Some standard requires tickers for currencies to start with an X. So, the usual abbreviation of Bitcoin, BTC, cannot be used in contexts where this standard is applied.
Usually, the unofficial ticker/abbreviation XBT is used in those contexts. Why? Bitcoin consists of two words, bit and coin, it's logical to abbreviate that to BC. For this reason XBC sounds like a much more logical ticker than XBT.
What is the first reference of XBT and is a consideration made before using it that favoured it above XBC?