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Hi,
The scale-in value actually isn't the target value, if you pull up the CloudWatch alarms created by the target tracking policy, the Low usage alarm will be some amount under the target value (dynamically calculated by target tracking). Additionally, it requires 15 periods of 60 seconds each to be triggered, so at the very least its not scaling in for 15 minutes (alarm settings aren't configurable with target tracking)
Additionally, target tracking has quite a few built in mechanisms to prevent excessive churn of your service. If target tracking doesn't feel like its 'safe' to scale-in, it won't. For example, if you have multiple target tracking scaling policies, it will only scale-in if all of them want to at once (ex, one for CPU and one for Memory Reservation is a common setup in ECS)
Target Tracking works quite well for the majority of usecases, but if you try it out and its not working for you, you can click the 'disable scale-in' option on the policy and make a custom Step Scaling based policy and alarm where you set everything to your exact preferred values. Just keep in mind that step scaling does NOT have all the other internal protections target tracking has to prevent churn. If the policy triggers and isn't blocked by cooldown/in-progress activities, it'll scale - so be careful when configuring all the alarm and policy settings.
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