Items marked (a) are active items; they look out for performance data from PerformanceGuard agents that actively contact specific servers at regular intervals to determine whether the servers are available, and how long it takes to get a reply from them. Items marked (p) are passive items; that is when PerformanceGuard passively waits for some computers to request something from a specified server/port combination and only collects performance data if requests are made. Items marked (t) are transaction items; they look out for performance data about web requests. Read more in Manage SLA Items.