Well, normally when you compress a folder with winrar, it'll compress one by one file until it reach it it limit then separate unfinished file to next part. For example, your folder have 24 files with same size and when compressed each file have 35 MBs, so the compressed file will be 840 MBs. If you set it limit 100 MBs each part then part1 include file 1st, 2nd, 3rd (30 MBs), part2 include file 3rd (5 MBs), 4th, 5th, 6th (25 MBs), part3 include file 6th (10 MBs), 7th, 8th, 9th (20 MBs) and so on until part9.
That's why if you put part1 in a folder and extract it, you got file 1st, 2nd, 3rd but file 3rd will get error because it missing next part, if you put part1 and part2 in folder and extract it, you got file 1st, 2nd, 3rd, 4th, 5th, 6th and file 6th will get error because it missing next part. So if you extract part 001 with only one error then that part mostly okay, and then if you extract part 1+part2 with multi error then pretty sure part 2 is the corrupted one.
With winrar, you can even know what file get separated, if you open a single part it'll show the arrow "<--" and "-->" at the separated files.
I'm not sure about 7zip though because you can't open single parts except the first part. But I think most compress software working same way.
