Stacks allows you to define global Terraform code that's shared across all stacks.
This can be used for use cases like defining a common Terraform state backend template (see this), or iterating through a list of regions to define multiple instances of the same provider (see this).
Global Terraform code must be located in the stacks directory like this:
|-- environments/
`-- stacks/
|-- ...
`-- global.tf # here
Any files named *.tf
will work.