This notification-only string carried in packet-mirroring failure traps gives the free-text error message as reported by the specific application on the E-Series router that encountered the failure. An admin reads this for additional diagnostic detail beyond what the enumerated juniPacketMirrorErrorCause conveys, especially useful when escalating to vendor support. For example, including the exact juniPacketMirrorErrorString text in a support case gives Juniper support the precise internal error message the failing application generated.