AWS Security group include another Security Group

I found my answer: Can't communicate between two ec2 instances in the same security group via public ip address?

Need to use private IP or whitelist the public IPs. I wish it was described in the documentation.

I was confused about what it exactly means to add another security group in Source (Inbound Rules) and Destination (Outbound Rules) when adding a new rule. I found the explanation given below (source: https://docs.aws.amazon.com/vpc/latest/userguide/VPC_SecurityGroups.html#SecurityGroupRules) very useful.

"When you specify a security group as the source for a rule, traffic is allowed from the network interfaces that are associated with the source security group for the specified protocol and port. Incoming traffic is allowed based on the private IP addresses of the network interfaces that are associated with the source security group (and not the public IP or Elastic IP addresses). Adding a security group as a source does not add rules from the source security group."