The key criterion is the timekeeping mechanism. Heading 9107 requires that the switching action be driven by a clock or watch movement or a synchronous motor — a mechanical or electromechanical element. Timers that use purely electronic means (quartz oscillator circuits, microcontrollers, or digital counters without a qualifying movement) are generally classified in Chapter 85 as electrical apparatus. Reviewing the internal mechanism's technical description is essential before selecting the heading.

If the synchronous motor serves as the actual timekeeping element that drives the switching function — rather than merely as an auxiliary motor in an otherwise electronic device — the article may still qualify under 9107. However, if the timing and switching logic is controlled by an electronic circuit and the synchronous motor is only a drive component, classification in Chapter 85 is more appropriate. The functional role of the synchronous motor within the overall timing mechanism is the deciding factor.