With Amazon CloudWatch, in addition to monitoring the built-in metrics that come with AWS, you can monitor with your custom metrics.
This new custom metrics feature can be used in two different ways:
1. You can add to the metrics collected for Amazon EC2 Instances, EBS Volumes, Elastic Load Balancers, and Relational Database Service DB Instances. The metrics that you store can be technical (system performance indicators) or business-related (user activity over the monitoring period).
2. You can store metrics for any generic resource. You can use CloudWatch to create a single, integrated storage and aggregation point for all of the metrics that you want to watch and to monitor.
In the exam (at least the test exam) you are asked to pick which ones are custom metrics.
Therefore, go through the available AWS metrics so you will get an idea of which metric is likely to be a custom metric when you are asked the question.
You can browse through the list of built-in metrics from the AWS web site here:
It will be hard to remember all the built-in metrics but you need to get some sort of idea as to what they are or how they are named so as to give you a chance of picking out a custom metric.