MTU (Monthly Tracked Users) is the number of unique customers you send a broadcast or automation message within your billing period (one month). This means that SMS, emails, Push notifications and Web in-app messages sent through Engage (web dashboard or API) count towards your MTU.
What we mean by “unique” is that if a customer receives more than one message in a month, the MTU is counted as one for that customer. Messages sent outside Engage, your transactional emails for example, do not count towards your MTU.
Your MTU is available on your account dashboard and in Settings » Billing.