Random vibration contains a spectrum of frequencies. Stationary random vibration has the same spectrum of frequencies over time. The spectrum of frequencies is characterized by the PSD table that is the input to the random vibration analysis. The output is a statistical result. If you are interested in the maximum value of acceleration rather than the 3 sigma value, that depends on how long the test runs. The longer the duration of the test, the higher will be maximum value of acceleration.
Harmonic Response analysis typically uses a fixed value of acceleration swept over a frequency range. At each frequency, the input is a pure sine wave at that frequency and amplitude. Therefore, the output response is not statistical, it is a single value at each node. In the plots below, the input acceleration was 2 mm/s^2.
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