service level

This expression is compositional rather than a term of art. In the quoted context, "service level" is an ordinary architectural qualifier — one layer in a stack (alongside the application level, network level, etc.) — indicating the stratum at which a security control or mechanism is applied. The field's genuine defined term of art is the full compound "service level agreement (SLA)," which NIST, ISO, and other bodies define precisely as a formal commitment between a service provider and customers specifying performance, reliability, and security expectations.