Default and Nondefault VPCs
AWS supports two types of accounts, one is EC2-classic, and the other is EC2-VPC. Accounts created after 2013 supports EC2-VPC, and the older accounts are EC2-classic platforms.
Default VPCs
If your account supports the EC2-VPC platform, it comes with a default VPC with default subnet in each availability zone. An instance would launch in the default VPC if you did not specify a subnet.
Nondefault VPCs
You can create and configure your VPC regardless of the which platforms your account supports which are known as nondefault VPCs.
The additional subnets created in default VPCs and subnets present in nondefault VPCs are called as nondefault subnets.
