Mobile terminated, or MT, describes a message sent from a business to a customer's phone. It is the outbound direction, and the one that is billed, registered and filtered.
What an MT message carries
Over SMS an MT message is text, split into 160-character segments if it runs long; see the RCS character limit for how the two channels compare. Over RCS an MT message from a verified RBM agent can be text, a rich card, a carousel or a file, with suggested replies and actions attached, and it arrives under the brand's name and logo rather than a phone number.
What has to be true before it can send
Every MT message needs prior consent, an appropriate sender (a launched RCS agent, and a registered 10DLC or toll-free number for the SMS fallback) and, for marketing, respect for quiet hours. Carriers apply spam filtering to MT traffic, not to replies, which is why unregistered outbound volume gets blocked while a customer's reply never does.
The return path
Anything the customer sends back is mobile originated. Delivery and read receipts for an MT message also travel back as MO events, which is how a platform reports delivery rate.