The stack version defines the collection of resources, their configuration and dependencies, used to form your environment's infrastructure. Stack versions are defined at the project level and are inherited by environments when they are created. As a result; new infrastructure related features may not be supported by older stack versions.
When creating a new project, the latest available stack version is selected automatically. Environments created inside a project will always use the latest patch of the stack version that is set.
Stacks versions that are supported receive ongoing security and bug patches.
Stack versions that enter maintenance no longer receive ongoing security and bug patches.
The default stack version for projects created today is 5.1.0. You can discover the stack version your project is using by going to Project → Settings → General.
Contact your customer success manager to find out how you can upgrade your stack version.