Amazon EMR now supports using multiple custom Amazon Machine Images (AMI) when you mix Arm-based AWS Graviton2-based instances with non-Graviton2 based instances in a single cluster. This capability allows you to diversify across more instance types when using custom AMIs, helping improve your access to EC2 capacity for large clusters. Prior to this release, you could still mix multiple instance types within a cluster, but could not do so when using Custom AMIs. Custom AMIs enable you to preload additional software and libraries required by your applications, customize cluster and node configurations, and encrypt the EBS root device volumes of EC2 instances in your cluster.
Source:: Amazon AWS