Explanation:
Box 1: Service bus queue
You are developing a back-end Azure App Service that scales based on the number of messages contained in a Service Bus queue.
Box 2: ActiveMessage Count
ActiveMessageCount: Messages in the queue or subscription that are in the active state and ready for delivery.
Box 3: Average
For special metrics such as Storage or Service Bus Queue length metric, the threshold is the average number of messages available per current number of instances.
Box 4: Less than or equal to
You need to add a new rule that will continuously scale down the App Service as long as the scale up condition is not met.
Box 5: Decrease count by
Reference:
https://docs.microsoft.com/en-us/azure/azure-monitor/autoscale/autoscale-best-practices#considerations-for-scaling-threshold-values-for-special-metrics
https://docs.microsoft.com/en-us/azure/service-bus-messaging/message-counters