Server extension feature is available only for users who have signed the service contract. See Feature termination for trial users (2021.01.19) for details.
Though server code has some limitations for security including the number of executable steps, it allows you to freely access external servers and data on Kii Cloud via the Kii Cloud SDK. Server code allows flexible implementations when a mobile app cannot cover your requirements.
Executing server code does not decrease the security of Kii Cloud because server code runs in the sandbox. You do not need to be responsible for the server security because Kii maintains the execution environment.