FortiGate performance SLAs monitor the state of each member-whether it is alive or dead-and measures the member packet loss, latency, and jitter.
When you configure a performance SLA, you can decide whether you want to monitor the link health actively or passively. In active monitoring, the performance SLA checks the health of the member periodically-by default every 500ms- sending probes from the member to one or two servers that act as a beacon. In passive monitoring, the performance SLA determines the health of a member based on the traffic passing through the member.
The SLA target section is optional. It's where you define the performance requirements of alive members (latency, jitter, and packet loss thresholds). The performance SLA uses SLA targets with some SD-WAN rule strategies, like Lowest Cost (SLA), to decide if the link is eligible for traffic steering or not.