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.
Note that if you are building a simple automation for your own private use only through your website on Crisp, you may use the simpler Web Hooks instead. Web Hooks can also be used from plugins as a first class alternative to the RTM API.