After acquiring another company and its AWS accounts, Cost Explorer shows that 20% of costs are attributed to "No Tagkey." How should you assign tags to those untagged resources so costs can be allocated properly?

The AWS Resource Groups Tag Editor is the most effective tool for this scenario. It allows you to search for resources across multiple AWS accounts and Regions based on specific tag keys or values, or to find resources that are missing certain tags. Once identified, you can then apply, edit, or remove tags in bulk, which is ideal for retroactively tagging a large number of untagged resources from acquired accounts. Adding accounts to AWS Organizations and using an SCP to force tags only applies to new resources or modifications, not existing untagged resources. An AWS Config rule with a termination action is destructive and not suitable for simply tagging resources. Cost Explorer is for analyzing costs, not for directly managing resource tags.