In ISA/IEC 62443, Target Security Protection Ratings correspond to the concept of Target Security Levels (SL-T).
Step 1: Definition of target ratings
Target ratings describe the desired level of protection that a system or zone must achieve during operation, based on risk assessment.
Step 2: Difference between target and achieved
* Target ratings define what is required.
* Achieved ratings (SL-A) describe what has actually been implemented and verified.
Step 3: Why Option D is correct
The target rating outlines the intended security outcome that implementation measures must fulfill during operation, not what currently exists.
Step 4: Why other options are incorrect
Actual achieved levels, cost metrics, or implementation measurements are separate concepts.
Therefore, the correct description is that they outline the desired levels of system security requirements to be fulfilled during operation.