I have a set of RAR-files, named .rar, .r01, .r02 to .r81. When I right-click on the .rar-file and choose Extract, it extracts just fine. However, when I right-click on a .r00 or .r01 file and choose Extract, I get an error (translated): "This is an incomplete multi-volume archive. Parts may be missing or you're not opening the first part."
When I try this with WinRar it extracts fine, no matter what part of the set I use to start the extraction.
Although it's a minor thing, it sure would make extracting rar-sets a lot quicker when I can just click on any file in the set.
That is intentional (not a bug) and won't be changing.
WinRAR allows you to (usually accidentally) open something other than the first volume of a multi-volume set, and then see only the files in and after that volume, skipping the ones in the volumes before it.
Opus, on the other hand, protects you from doing that and will only let you open the first volume. (In the very rare cases where the earlier volumes are missing and you want to salvage the remaining files, you can use another tool.)
(I know that WinRAR lets you right-click a later part and choose extract to extract the whole thing, including earlier parts, but that makes it even more confusing IMO. It's different to what happens if you right-click the exact same part and choose Open with WinRAR, for example. You have to be careful which operations you do on which parts.)
There are some other reasons we don't allow this, too. For example, if Opus allowed you to open the other parts of a multi-volume archive then if you did a Find in archives you would get a duplicate set of results for each part of the archive.