ETS uses carbon credits to limit greenhouse gas emissions. For example, if you produce more than 25Mg of CO2 equivalent per passenger per year (about 5kg), you must buy back your own unused credits or pay a fine. This is often a financial burden for companies. ETS does this automatically. If you don’t use all your allowances within 2 years, you lose them. You have to prove you are using all your allowances, otherwise you face penalties. If you fail to use any allowance at all, you lose them forever. I wanted to verify how many passengers my flights had so I could sell the ones I didn’t need. My carrier has a system called the Aircraft Logistics Management System (ALMS) and provides a ‘passenger manifest’ with all the information about each flight, including its start date and end date. But this is not enough, since I only need the total number of passengers carried over a certain period of time. The EU-ETS database contains all the information about every flight ever operated by every airline in Europe. It is used to monitor airlines' compliance with the rules. As long as they comply, they receive free credits.