Disk Cleanup system files, Safe?

Was trying to optimize my computer with this thread, Disk Cleanup - Open and Use

I cleared my unnecessary non-system files, but Im worried about clearing my unnecessary system files, as it doesnt really say what its gonna delete.

The things Im worried about are stuff that I installed into the windows and system32 directory, some video codec .dlls, audacity/goldwave libraries, photoshop/after effects plugins, visual basic/.net framework files

So my question is
what will disk cleanup delete?
Will those that I listed be affected in anyway?
and how much does it normally free? (if its below 1gb Im probably gonna skip it)
does it fix errors or makes my comp faster in anyway or just save up space?

Thanks in advance

In previous iterations of windows I used to do this manually. For the last 2 + years I have used this feature without incident. If you choose system files service pack backup file(s) will be deleted and this could free up 500 + MB. None of your listed files will be zapped. Only space will be freed up AFA I know. The first time I ran it and used the system file option I did do a full system image just in case of problems but I have not run into any many months later.