P4000 pedestal server hot-swop drives

hi

Can someone enlighten what is the difference between the models of pedestal server hot-swop drives below?

FUP4X35HSDK

FUP4X35S3HSDK

FUP8X35HSDK

FUP8X25HSDKS

FUP8X25S3HSDK

For a start, I believe the front portion of the model number like FUP4X35 / FUP8X25 would mean the number of hot-swop drive trays and the physical size of the hdd.

But what does the the rest of the letters mean? "S3", "S" at the end of the model, etc. Is there a page that helps to explain?

thank you