An assessment boundary is the defined set of systems, applications, users, networks, cloud services, and connected environments that are included in a compliance review. In frameworks such as CMMC, the boundary shows exactly what is in scope for controls, testing, and evidence collection. It is not a company-wide promise of security; it is a formal line around the assets that must meet the required safeguards.
This matters because attackers often look for weak points just outside or across the boundary, such as shared identities, unmanaged endpoints, third-party integrations, or cloud misconfigurations that can bridge into in-scope systems. Defenders use the boundary to focus hardening, logging, access control, and asset inventories on the right environment, and assessors use it to verify that the documented scope matches reality. If the boundary is drawn too broadly, compliance becomes expensive; if it is drawn too narrowly or inaccurately, sensitive data may be left unprotected.