A data-processing boundary is the point where information moves from one trust zone to another, such as from an employee laptop into a cloud API, from a company app into a vendor service, or from one internal system into another. It marks where a different party or control plane begins handling the data, and that change can affect privacy, retention, logging, residency, and access rights.
In cyber security, boundaries matter because they define where sensitive material can leak or be overexposed. Prompts, source code, credentials, and telemetry may cross a boundary when teams use AI assistants, SaaS tools, or managed platforms. Defenders use boundary mapping to decide what data may leave the environment, which fields must be masked, and where audit logs should be kept. Clear boundaries support least privilege, contractual controls, and incident response, while vague ones make it easier for attackers or misconfigurations to expand the blast radius.