This parameter is required for Amazon ECS tasks hosted on Fargate. See Runtime platform for more details.

When you register a task definition, you specify the CPU architecture. The valid values are X86_64 and ARM64.
When you have Linux tasks for either the Fargate launch type, or the EC2 launch type, you can set the value to ARM64.
For more information, see Working with 64-bit ARM workloads on Amazon ECS.