The second setting is for alarms based on metric percentiles that might have small number of data samples. By default, CloudWatch Alarms evaluate percentile values irrespective of the sample count, but now you have the option to ignore percentiles with low data samples and maintain the alarm state. The new alarm settings can be applied on any CloudWatch Alarms through the alarms console or PutMetricAlarm API.

To learn more about these new CloudWatch Alarms settings, please see the documentation here.