In Terraform Cloud, speculative plan runs are not automatically started when changes are merged or committed to the version control repository linked to a workspace. Instead, speculative plans are typically triggered as part of proposed changes in merge requests or pull requests to give an indication of what would happen if the changes were applied, without making any real changes to the infrastructure. Actual plan and apply operations in Terraform Cloud workspaces are usually triggered by specific events or configurations defined within the Terraform Cloud workspace settings.References= This behavior is part of how Terraform Cloud integrates with version control systems and is documented in Terraform Cloud's usage guidelines and best practices, especially in the context of VCS-driven workflows.

Explanation
A module cannot always refer to all variables declared in its parent module, as it needs to explicitly declare input variables and assign values to them from the parent module's arguments. A module cannot access the parent module's variables directly, unless they are passed as input arguments.

Not all standard backend types support state locking and remote operations like plan, apply, and destroy. For example, the local backend does not support remote operations and state locking. State locking is a feature that ensures that no two users can make changes to the state file at the same time, which is crucial for preventing race conditions. Remote operations allow running Terraform commands on a remote server, which is supported by some backends like remote or consul, but not all.
References:
Terraform documentation on backends: Terraform Backends
Detailed backend support: Terraform Backend Types

These are features of Terraform Cloud, which is a hosted service that provides a web-based UI, remote state storage, remote operations, collaboration features, and more for managing your Terraform infrastructure.

Terraform validate reports configuration consistency errors, such as declaring a resource identifier more than once. This means that the same resource type and name combination is used for multiple resource blocks, which is not allowed in Terraform. For example, resource "aws_instance" "example" {...} cannot be used more than once in the same configuration. Terraform validate does not report errors related to module versions, state differences, or formatting issues, as these are not relevant for checking the configuration syntax and structure.
References = [Validate Configuration], [Resource Syntax]