What is indicated on a waterfall display by one or more vertical lines on either side of a data mode or RTTY signal?

In digital modes like RTTY or PSK, overdriving the audio input to the transmitter causes overmodulation. This results in signal distortion and the generation of spurious emissions, often referred to as "splatter." On a waterfall display, this splatter appears as extra, unwanted vertical lines running parallel on either side of the main signal. This visually indicates that the signal is taking up more bandwidth than necessary. Common distractors like insufficient modulation would simply make the main signal faint, not create extra lines. Propagation effects like long path or backscatter might cause fading, echo, or smearing, but do not create distinct, sharp vertical lines adjacent to the primary frequency.