It is described in this site described in the bitcoin site "Bitcoin - with capitalization, is used when describing the concept of Bitcoin, or the entire network itself. e.g. "I was learning about the Bitcoin protocol today."bitcoin - without capitalization, is used to describe bitcoins as a unit of account. e.g. "I sent ten bitcoins today."; it is also often abbreviated BTC or XBT."

Source: https://bitcoin.org/en/vocabulary#bit

The sentence used by @pamoldar is a great example of how to and when to use Bitcoin with a capital B and with a small letter b.