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

Software development encourages code reuse through reusable artifacts, such as libraries, packages and modules. Most programming languages enable developers to package and publish these reusable components and make them available on a registry or feed. For example, Python has Python Package Index and PowerShell has PowerShell Gallery.
For Terraform users, the Terraform Registry enables the distribution of Terraform modules, which are reusable configurations. The Terraform Registry acts as a centralized repository for module sharing, making modules easier to discover and reuse.
The Registry is available in two variants:
* Public Registry houses official Terraform providers -- which are services that interact with an API to expose and manage a specific resource -- and community-contributed modules.
* Private Registry is available as part of the Terraform Cloud, and can host modules internally within an organization.
https://www.terraform.io/docs/registry/index.html