Build - Subnet Group Details
When you select a Subnet Group on your template, you will see a panel with details of this service on right side. A Subnet Group is a collection of subnets (typically private) that you can designate for your clusters running in an Virtual Private Cloud (VPC) environment. Usually for Databases - Relational Database Service (RDS).
Name: The name of the Subnet Group (It depends on if you are using Naming Convention)
Description: Description of the Subnet Group.
To change the group of subnets you can click on Change (as shown in image above) or click on Edit (as shown in image below).
It will be open the following window. You can choose all subnets for this group. Then click on Done to conclude.