The RTM API lets integrations receive real-time notifications of all events happening in Crisp websites it has access to. This can be used in pair with the REST API to eg. send a message through the REST API, and receive responses from the other party through the RTM API.
The RTM API uses WebSocket. It lets you establish a single connection, used to receive all events happening on multiple subscribed websites. Note that RTM API capabilities are included in most of our REST API libraries.