The public Terraform Module Registry automatically exposes all the information about published modules, including required input variables, optional input variables and default values, and outputs. This helps users to understand how to use and configure the modules.

In Terraform, a data block is used to fetch or compute information from external sources for use elsewhere in the Terraform configuration. Unlike resource blocks that manage infrastructure, data blocks gather information without directly managing any resources. This can include querying for data from cloud providers, external APIs, or other Terraform states.
Reference = This definition and usage of data blocks are covered in Terraform's official documentation, highlighting their role in fetching external information to inform Terraform configurations.

This is not a valid Terraform variable type. The other options are valid variable types that can store different kinds of values2.

The reason why the apply fails after adding a new provider to the configuration and immediately running terraform apply in the CD using the local backend is because Terraform needs to install the necessary plugins first. Terraform providers are plugins that Terraform uses to interact with various cloud services and other APIs. Each provider has a source address that determines where to download it from. When Terraform encounters a new provider in the configuration, it needs to run terraform init first to install the provider plugins in a local directory. Without the plugins, Terraform cannot communicate with the provider and perform the desired actions. Reference = [Provider Requirements], [Provider Installation]

This is a key benefit of the Terraform state file, as it stores and tracks the metadata and attributes of the resources that are managed by Terraform, and allows Terraform to compare the current state with the desired state expressed by your configuration files.