Machines
A machine is a computer that runs the infraternal agent, the program that lets Infraternal open terminals on it.
What is here
You add a machine from the app: the server connects to it over SSH and gives it a join token that works once. From then on the machine proves who it is with a certificate that renews by itself while it is online. See Adding and trusting hosts for each token and certificate and how long it lasts.