Disk optimization and disk defragging

I just installed the trial version of NS. One feature which I don't clearly understand either reading about it in the manual or talking with a "chat expert" (who had no idea how to answer this) is the disk optimization option.

If you look under Tasks Scheduling, the last option is disk optimization. It says it "rejoins fragmented files". I can only assume this is different from disk defragmentation as you do not want to defrag a SSD and it comes pre checked (or at least mine did).

Can someone please explain if it is different and do I want to leave it checked if my C drive is a solid state drive? My other drives are mechanical HD's.

Thanks...