Hi @giakonda,
zim files are the format that Wikipedia releases their full copies of content as. In the past they were split into multiple files, for example en-wikipedia_2017.zima, en-wikipedia_2017.zimb, en-wikipedia_2017.zimc, etc. These files are now released as one single .zim file, so en-wikipedia_2017.zim.
Here’s a bit more info about how that affects RACHEL. There are a set of tools called kiwix-tools that RACHEL uses to serve wikipedia content ( kiwix-serve ) and manage the Kiwix library ( kiwix-manage ) by adding zims to a library.xml file. Zim files previously used separately generated index files for the search functionality. The latest versions of those tools no longer take a provided index anymore when adding zims to the library with kiwix-manage. Older zims ( .zima, zimb, etc ) can be added but since kiwix-tools doesn’t take the index, they won’t be indexed properly for search with the new kiwix-tools. So in the near future I will update kiwix-tools to the latest version and create a way to directly generate wikipedia modules for RACHEL based on a .zim file.
Hope that explains it
James