GNU/Linux distributions are highly heterogeneou, the control of the system platform is a complex task; expertise becomes really necessary as the number of services and servers increases.
However, some hardening measures can be implemented based on the expected security level, which will depend on the sensitivity of the data handled or hosted by the system and the robustness of access controls realized in order to access to resources.
A publicly exposed service with low access control and handling sensitive data (transfer email server, corporate web server, etc.) requires an enhanced security level, perhaps even high. Conversely, a backup server on an isolated network and accessible only to a few people may require a lower level of security.
The recommendations of this guide are given based on an estimated level of hardening.