I did not find what the BTC abbreviation is reserved for.
Based from advanced search results:
Bitcoin was originally abbreviated by satoshi as "
BC" which is straightforward 'Bit' & 'coin'; he also used it for the unit, but he started using "
BTC" after a while.
Some still used BC as an abbreviation for 'Bitcoin' in 2010~11, then BTC for the 3-letter unit. There were also a lot of topics concerning the unit of currency.
But it came down to "
standardness", means BTC has been the most used unit and abbreviation.
(IDK if there's an official announcement to use BTC since I can't find any, anyone?)