Terraform is built on a plugin-based architecture. All providers and provisioners that are used in Terraform configurations are plugins, even the core types such as AWS and Heroku. Users of Terraform are able to write new plugins in order to support new functionality in Terraform.

A terraform list is a sequence of values identified by consecutive whole numbers starting with zero.
https://www.terraform.io/docs/configuration/types.html#structural-types

You can use required_version to ensure that a user deploying infrastructure is using Terraform 0.12 or greater, due to the vast number of changes that were introduced. As a result, many previously written configurations had to be converted or rewritten.

Input variables serve as parameters for a Terraform module, allowing aspects of the module to be customized without altering the module's own source code, and allowing modules to be shared between different configurations.

To extend Vault beyond a data center or cloud regional boundary, replication can be used. Vault supports both DR replication and Performance replication to copy data from the primary cluster to a secondary cluster safely.