Reference: https://cloud.google.com/kubernetes-engine/docs/how-to/creating-a-cluster-windows

You can optionally define multiple configurations for the same provider, and select which one to use on a per-resource or per-module basis. The primary reason for this is to support multiple regions for a cloud platform; other examples include targeting multiple Docker hosts, multiple Consul hosts, etc.
To include multiple configurations for a given provider, include multiple provider blocks with the same provider name, but set the alias meta-argument to an alias name to use for each additional configuration. For example:
# The default provider configuration
provider "aws" {
region = "us-east-1"
}
# Additional provider configuration for west coast region
provider "aws" {
alias = "west"
region = "us-west-2"
}
https://www.terraform.io/docs/configuration/providers.html

Terraform Registry is an index of modules shared publicly using this protocol. This public registry is the easiest way to get started with Terraform and find modules created by others in the community.