When archive is created with parts, you have 1 file with .zip or .rar extension and part files ending in numbers like .z01. Depending on the options selected, you may be able to extract some files from an incomplete archive or not. To have all the files, you will need all the parts of the archive and also the .zip file. You can see the process - take a 12 MB file, compress with parts of size 5 MB. You will get .zip and 01 and 02 files.
BTW, have you download the files? I was downloading selected course files and the host has gone down with me having partial files now. Do you have Sec545 Video - @Hide01 @SansDefense.z01 file and can share?