Work in progress page incomplete.
PAR files come in 2 different versions.. par 1 files and par2's the way to tell which is used for a perticular set of files is the file name extension. Below is a selection of "par2" files par 1 files are easy spot also as the typical file extension for par1 files will be...
PAR, .P01, .P02, .P03, and so on (much shorter file names
Par 1 files are hardly used these days the reason for this is that par 1 files replace a missing or incomplete file so if you have 3 missing/incomplete files you will need 3 par1 files to repair them regardless of how much is missing par 1's are just a like for like swap of files. Par2 files on the other hand contain a certain amount of "recovery blocks" per par file this makes them far more user friendly as you may have 20 incomplete files that just have 2 blocks missing each. To recover/repair these 20 files all you would need is 40 recovery blocks so instead of needing 20 par files to fix the problem you would just need to grab maybe two par files that contained 20 blocks each or even 1 par file that contained 40 blocks. For this reason par 2 files are far more versitile (its even possible to repair a set of rars that are all incomplete with this method with a few par files)
Here is a typical bunch of par2 files, all you really need to pay attention to here is the part i have highlighted in green, the highlighted part shows the amount of recovery blocks each par file contains. Here we have 200 recovery blocks.