Instruments and apparatus specially designed for telecommunications testing fall under 903040. A network analyzer used primarily to characterize RF and microwave components in a telecommunications context would typically go there. If the instrument is a general-purpose vector network analyzer used across multiple industries, classification may depend on its principal design purpose. Manufacturer documentation and the instrument's feature set relative to telecommunications-specific functions guide the determination.

Subheading 903082 is reserved for instruments and apparatus specially designed for measuring or checking semiconductor wafers or devices — for example, wafer probers, parametric testers, and in-line metrology tools used in chip fabrication. General-purpose electrical instruments that happen to be used in a semiconductor lab but are not specially designed for wafer or device testing remain under the appropriate general subheading. The 'specially designed' criterion is the classification pivot.

A clamp meter that measures current, voltage, and resistance functions as a multimeter and would generally classify under 903031 (without recording device) or 903032 (with recording device). A dedicated insulation tester (megohmmeter) that measures only resistance or insulation resistance may fall under 903033 or 903039 as an instrument for measuring resistance without a recording device. The number of measurable quantities and the presence of a recording function are the key classification variables.