14 C.F.R. § 1203.402
Items of equipment or other physical objects may be classified only where classified information may be derived by visual observation of internal or external appearance, structure, operation, test, application or use. The overall classification assigned to equipment or objects shall be at least as high as the highest classification of any of the items of information which may be revealed by the equipment or objects, but may be higher if the classifying authority determines that the sum of classified or unclassified information warrants such higher classification. In every instance where classification of an item of equipment or object is determined to be warranted, such determination must be based on a finding that there is at least one aspect of the item or object which requires protection. If mere knowledge of the existence of the equipment or object would compromise or nullify the reason or justification for its classification, the fact of its existence should be classified.
14 C.F.R. § 1203.402