Final answer:
The highest classification allowed for performance report supplements with unavoidable classified comments is C2. This limits dissemination of classified information to only those who need it for their duties.
Step-by-step explanation:
The highest classification allowed for performance report supplements, when classified comments are unavoidable, is C2. This designation indicates that while the primary document may be of a higher classification, any supplementary comments must not exceed the C2 classification level to ensure that sensitive information is adequately protected but still accessible to those with the appropriate clearance level. The aim is to limit the dissemination of sensitive information only to individuals who require access to perform their duties effectively.