The Auto Spectral Density or Auto RMS spectrum analyses uses Fourier Transforms to process optionally overlapped sections of the input data. The result of each Fourier analysed section is called a periodogram. We then process all the resulting periodograms to produce a spectral result.
This is just the ensemble average of all the periodograms. Basically all the periodograms are summed together on a point for point basis and the average value at each point is the standard spectral result
This finds the ensemble maximum level rather than the average value. All the periodograms are compared with each other on a point for point basis. The maximum value at each point is the Limit Hold spectral result.