A solid archive treats all files within the archive as one continuous data stream. This is specifically useful for sets like your "part6.rar" because:
: To extract a single file from a solid archive, all preceding files in that stream must be analyzed first.
If you are dealing with a multi-part archive (like part6.rar of the SD Gundam Battle Alliance update v1.30), keep the following in mind:
: Ensure you have all parts (part1 through part6) in the same folder before starting the extraction process with a tool like WinRAR or 7-Zip. Solid archives - WinRAR - Documentation & Help
: If one part of a solid volume set is damaged, it can prevent the extraction of all subsequent files unless a recovery record was included by the creator.