In Terraform Cloud, you can switch between workspaces using both the web UI and CLI. The statement that you "must use the CLI" is false. Workspaces can securely store cloud credentials, offer role-based access control, and integrate with VCS to trigger plan and apply operations.
Reference:
Terraform Cloud Workspaces

The Terraform Cloud private registry provides the ability to restrict modules to members of Terraform Cloud or Enterprise organizations. This allows you to share modules within your organization without exposing them to the public. The private registry also supports importing modules from your private VCS repositories. The public Terraform Module Registry, on the other hand, publishes modules from public Git repositories and makes them available to any user of Terraform. References = : Private Registry - Terraform Cloud : Terraform Registry - Provider Documentation

The terraform taint command marks a resource as tainted, which means it will be destroyed and recreated on the next apply. This way, you can replace the VM instance without affecting the database or other resources.
References = [Terraform Taint]

In Terraform, when you use the count meta-argument, you can reference individual instances using an index. The indexing starts at 0, so to reference the "name" value of the second instance, you would use aws_instance.web[1].name. This syntax allows you to access the properties of specific instances in a list generated by the count argument.
References:
Terraform documentation on count and accessing resource instances: Terraform Count

Explanation
This command will check if all code in a Terraform configuration that references multiple modules is properly formatted without making changes, and will return a non-zero exit code if any files need formatting. The other commands will either make changes, list the files that need formatting, or not check the modules.