I suspect that when the Amplitude is set to "no", Mechanical is reporting the maximum S1 over the frequency range at some fixed phase angle (the default is probably zero - the real part of the response). When you set Amplitude to "yes", it likely determines the maximum amplitude of S1 over a full cycle of vibration (instead of at a particular phase angle). It then does this at each frequency of the harmonic sweep and reports the maximum values at all locations in the model, which I believe is what you've been asked to determine.
Kind regards,
Bill
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