The statistic's unit of measure. For example, the units for the Amazon EC2 NetworkIn metric are Bytes because NetworkIn tracks the number of bytes that an instance receives on all network interfaces. You can also specify a unit when you create a custom metric. Units help provide conceptual meaning to your data. Metric data points that specify a unit of measure, such as Percent, are aggregated separately.
Note: If you specify a unit, you must use a unit that is appropriate for
the metric. Otherwise, this can cause an Amazon CloudWatch alarm to get stuck in the INSUFFICIENT DATA state.