Hi @seungwoo_hwang and welcome to MongoDB community forums!!
Based on the above statement, I would like to outline my interpretation with the below example.
You have an Atlas cluster created in region 1, and you have three EC2 instances running in the same region on the AWS account. Considering that a VPC connection is set between both, according to the above statement, you would like to establish connection between Atlas and the AWS such that only one or two of the EC2 instances but not all, are able to connect to the Atlas Cluster.
Could you please confirm if my understanding is correct. It would highly appreciated if you could clarify based on an example situation.
Ideally, when you establish a VPC connection between Atlas and AWS instances, establishing a security group within the resource becomes one of the prerequisite to make the connection between them.
Also, can you also help me with a detailed clarification for the above statement.
Regards
Aasawari