If the article's principal function is timekeeping and it is of a type designed for instrument-panel mounting in aircraft, 9104 is the appropriate heading. However, if the GPS module's primary function is navigation or data processing and the clock is incidental, classification may shift to Chapter 85 or Chapter 90. The principal-function test and the Chapter 91 exclusion notes should be applied carefully, and a binding ruling may be warranted for complex avionics assemblies.

Heading 9104 specifically captures instrument-panel and similar clocks designed for vehicles, aircraft, spacecraft, or vessels, and it takes precedence over 9103 for those articles. Heading 9103 covers clocks with watch movements that are excluded from 9104 — meaning a clock with a watch movement that is not of the instrument-panel type for a vehicle would go to 9103, while one designed for dashboard integration goes to 9104 regardless of movement type.