DB subnet groups:
While creating DB, you need to pick a DB subnet group for the DB instance to reside in.
A DB subnet group is a collection of subnets (typically private) that you create in a VPC and that you then designate for your DB instances.
Each DB subnet group should have subnets in at least two Availability Zones in a given AWS Region.
The DB instance uses the Availability Zone that contains the subnet.
If the primary DB instance of a Multi-AZ deployment fails, Amazon RDS can promote the corresponding standby and subsequently create a new standby using an IP address of the subnet in one of the other Availability Zones.
The subnets in a DB subnet group are either public or private. They can’t be a mix of both public and private subnets. The subnets are public or private, depending on the configuration that you set for their network access control lists (network ACLs) and routing tables.