PC-based or software-defined instruments — where the measurement function is performed by a standard computer running specialized software with a plug-in hardware module — present classification challenges. CBP generally looks to the essential character of the article; if the hardware module is the functional core, it may classify as a measuring instrument, but if the PC dominates, it could classify as an ADP machine. A binding ruling request to CBP is advisable for novel instrument architectures.

Test and measurement equipment that intentionally or unintentionally emits radio-frequency energy may require FCC equipment authorization before it can be marketed or used in the US. Many professional-grade instruments qualify for exemptions under FCC Part 15 rules for test equipment, but importers should confirm the applicable exemption or authorization category with an FCC regulatory specialist, particularly for instruments that generate RF signals as part of their test function.