What is second BTC?

Excuse me, could you elaborate on the term "second BTC"? I'm not entirely sure what you mean by that phrase, as "BTC" is commonly used as an abbreviation for Bitcoin, the world's largest and most widely adopted cryptocurrency. Are you referring to a specific altcoin or cryptocurrency that aims to be a successor or alternative to Bitcoin? Or perhaps you're referring to a second layer solution built on top of the Bitcoin blockchain? Clarification would be greatly appreciated.