FEC91 (Forward Error Correction) is a method used in networking to improve the reliability of data transmission over a network. It is often used in high-speed data links, such as those used in data centers and other mission-critical environments. FEC mode in JunOS is a feature that allows the router to detect and correct errors in the data transmission. It is enabled by default, but in some cases, it may be necessary to disable it.

systemd resource accounting is a feature that allows you to monitor and control the usage of system resources, such as CPU and memory, by individual services and processes. One of the most common types of resource accounting is CPU accounting, which allows you to measure the amount of CPU time used by a specific service or process. This is accomplished by using the CPUAccounting=true option in the service’s systemd unit file.