Solid compression is most effective when you are archiving a large number of very similar files, such as multiple versions of a document or software backups. Instead of compressing each file individually, 7-Zip looks for redundancies across all files in the "solid block."
7-Zip / Discussion / Help: 7-zIp And Compression Noob - SourceForge Elle.7z
: While it saves space, updating or extracting a single file from the middle of a large solid archive can be slower, as the software may need to decode the preceding data first. Performance & Configuration Tips Solid compression is most effective when you are