A "mix" list means the credentials come from multiple different email providers (e.g., Gmail, Yahoo, Outlook) rather than a single source. Cybercriminals use these .txt files to:
: Use reputable services like Have I Been Pwned to see if your email address has appeared in known data breaches.
: Compromised accounts are often used to send spam or phishing emails to the victim's contact list, making the scams appear more legitimate.