Okay this what AWS Website says on these topics:
[T2 instances] The number of earned CPU credits that an instance has accrued since it was launched or started. For T2 Standard, the CPUCreditBalance also includes the number of launch credits that have been accrued.
Credits are accrued in the credit balance after they are earned, and removed from the credit balance when they are spent. The credit balance has a maximum limit, determined by the instance size. Once the limit is reached, any new credits that are earned are discarded. For T2 Standard, launch credits do not count towards the limit.
The credits in the CPUCreditBalance are available for the instance to spend to burst beyond its baseline CPU utilization.
When an instance is running, credits in the CPUCreditBalancedo not expire. When the instance stops, the CPUCreditBalancedoes not persist, and all accrued credits are lost.
CPU credit metrics are available at a five-minute frequency only.
Units: Credits (vCPU-minutes)