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When editing an archive it can only be saved in cbz, it would be nice if it could be saved in tar, rar and such
ZIP is a free and open file format, and it is possible (at least in theory) to access just one file without having to unpack and repack the whole archive.
RAR is a closed proprietry format. Developers are lucky to be have the unrar library so they can get files out this format. It is very unlikely that RAR support would ever be added.
TAR Gz , TAR Bzip2, or even 7zip might be possible, but it is likely they would be quite a lot slower than ZIP/CBZ.
(There's also the issue of Comix development not being active since 2009.)