Frequency Response Optimization

Hi all;

I performed related tutorial and i would like to kindly ask about frf-displacement response

there are sum sub options related with that..such as rss,ssq can anyone give me details of those suboptions?

I want to minimizie displacement of my plate like structure for the first 3 modes..targeting volume constrained of one layer of this plate..

Does ssq or rss works for this purposes? As fara as i searched they are related with sum of squares of ( applicable for first 3 modes ? ) arguments etc..

Any idea?