High clock rates, e.g. above 3 GHz approach 10 GHz already at the 3rd harmonic. These harmonics are decoupled from RF sources on the board, e.g. conductor sections, ICs and other components. Other structural parts of the assembly can be excited to oscillate and lead to interference emission.
With the high internal base frequency of today’s assemblies, the measurement of harmonic frequency multiples is an important step towards an EMC-compliant assembly.