XBT = iso notation.
Doesn't seem to be in the standard.
From
Wikipedia:
The following non-ISO codes are, however, sometimes used commercially. Usage of these non-ISO-codes is not portable
XBT 8 Bitcoin World-wide
Also
xe.com doesn't list it in ISO 4217:2008.
It's just mimicking ISO, from what I can tell it's just a way to appear
fancier in the financial world but BTC has been well established since long.
Also BTC sounds better.