As in potentially disruptive to critical-to-life public safety systems, e.g. air traffic control, emergency services communications, I suppose. Square waves =

As in potentially disruptive to critical-to-life public safety systems, e.g. air traffic control, emergency services communications, I suppose. Square waves = theoretically infinite harmonics at positive multiples of the fundamental frequency (in practice reducing with order of the harmonic) Then there's in-band spurs / spurious emissions, different from harmonics.