AWS Batch supported instance types per-region list?

I was going to make some AWS Batch compute environment in ap-northeast-1 region with i4i.large instance type just to be told the API that it's not a supported instance type.

Sure enough, going to the AWS Batch console and trying to create the compute environment manually, this instance type is not available in the drop-down list.

I do notice that this instance is available in us-east-1 which is a bit strange to me: surely it should also be available in ap-northeast-1, it's not a AWS Batch limitation... I can launch these instances fine as EC2 instances.

The questions are:

• Is there an API that can show list of available instance types for AWS Batch in a region? Or in all regions.
• Why are i4i instances not available in Batch in ap-northeast-1 even though it seems AWS Batch does have support for them in us-east-1?