A dependency boundary is the line that separates trusted core code from external components pulled into a build. In an operating system or application, the core may be maintained under stricter review, while libraries, tools, and packages outside that boundary are accepted only when they meet project policy, licensing, and security requirements.
This boundary matters because every imported component expands the software supply chain. The more code that crosses into the trusted core, the harder it becomes to audit provenance, track updates, and reason about attack surface. Security teams use clear dependency boundaries to keep critical builds predictable and to reduce the chance that an unnecessary package, outdated library, or risky transitive dependency becomes part of the foundation. In practice, boundary management shows up in base-system curation, vendoring decisions, and build rules that exclude components unless they are needed and reviewed.