According to Amazon's official documentation,
For Amazon ECS, AWS Fargate pricing is calculated based on the vCPU and memory resources used from the time you start to download your container image (docker pull) until the Amazon ECS Task* terminates, rounded up to the nearest second. A minimum charge of 1 minute applies.
Does this mean that
a) It knows when my containers consume cpu/ram and applies charges only for this time frames?
b) It aplies charges based on pre-allocations I do when creating the tasks?