Any lightning sent to Minibits goes to your active mint where it's held as ecash. When you send lightning from Minibits, it internally converts the required sats from ecash to lightning before sending. It's a unique system that I trust, but don't like how it works with multiple mints. That's why I only keep one active mint. If someone gives me ecash from a different mint, I'll usually keep that new mint active, but if it's a large sum or I anticipate holding it for a long time, I'll send it to Cashu and swap it to the mint I use in Minibits so I'm only dealing with that one mint.