I've seen since the early days some exchanges/people using the XBT tag, then I saw a lot of people using BTC. Nowadays i think BTC is the dominant tag. Anyway, whats the deal with this? I think BTC sounds better, XBT is confusing.
I can explain this.
BTC is prefered by many, because it sounds more like bitcoin, it just seems like a natural acronym.
But there's a problem.
In ISO standard, all 3 letter abbreviations for currencies have their first two letters be the country code (or region code), and the 3rd letter specifies which currency it is.
Some examples (some are old currencies):
NLG (Netherlands Guilder)
DEM (Deutsche Mark)
FRF (French Frank)
CHF (Swiss Frank) (country code for swiss is CH, don't ask me why)
CNY (Chinese Yuan)
JPY (Japanese Yen)
USD (United States Dollar)
CND (Canadian Dollar)
BRL (Brasilian Lire)
EUR (European Union Euro) (This one is a bit weird because it should be EUE, but this follows more like the 'BTC rule' of sounding cool)
And so on
So what do we do with currencies that aren't tied to a country?
Those get an X, like
XAU (Gold ounce (Au is gold on the periodic table))
XAG (Silver ounce)
and of course XBT (XBC was also suggested but lost out in popularity to XBT, (there's also already a currency using XBC))
BT is the country code for Bhutan, so BTC would be "Bhutan Coin"
Edit: It has been proposed by some to have XBT be the official tag for 1 µ
BTC (0.000001 bitcoin, or 100 satoshi) because this would be more compatible with most monetary standards that deal in currency and cents. (as there are 100 cents to an XBT). And the fear that some of those standards would break if they had to deal with tons of decimals.