During my using OpticStudio, sometimes the exact same imaging system gets different plots in sequential and non-sequential mode.

What's the difference between the 'image surface' of sequential and the 'detector' of non-sequential? 

And how can I get information about the imaging quality?(In sequential mode there are all kinds of tools such as Rays ＆ Spots, PSF in the image quality group. But in non-sequential mode there are only Trace Rays and Detector ＆ Analysis group. )

For example, how can I get the PSF information using detector?