A Commercial Learner's Permit (CLP) holder who has obtained the S endorsement but does not yet hold full CDL privileges must also carry which restriction?

AAMVA CDL Manual (nhcdm.pdf), Section 1, pp. 1-1–1-3

This restriction exists because CLP holders are still in training and have not yet passed the full skills test — understanding exactly who is and isn't allowed aboard protects both the trainee driver and any children who might otherwise be exposed to an unlicensed driver's training run.