Infrastructure
The layer applications run on: cloud servers, networks, and deployment systems, and the engineers who build and run it.
What it tells you about a developer
Infrastructure skill develops by running real systems at scale, so it is a strong seniority signal on its own. These engineers are scarce and senior-skewed, so expect outbound sourcing to carry the search. Someone who owns infra can usually write competent backend code as well.
Infrastructure engineers own the foundation under the product: setting up cloud resources, keeping services online, and automating deployment. Their tools are distinctive on a resume: Kubernetes, Terraform, Docker, Ansible, Helm, and one of the major clouds.