Archives are files that contain other files, such as .zip. Beyond Compare displays them as folders, with a zipper icon, but by default treats them as files when copying or comparing them. Once you have expanded the archive by double-clicking on it, the files within will be compared. Many of the formats support both reading and writing, so you can use all of the usual file operations on the files within the archives, including comparing the content, copying, renaming, etc.
Common archive formats
7z archive (*.7z;*.7z.001) 
BZip (*.bz;*.bz2)
BZipped Tar (*.tbz;*.tbz2;*.tar.bz2)
Beyond Compare Snapshot (*.bcss;*.bcs)
Compiled HTML Help (*.chm)
Deb archive (*.deb) 
GZip (*.gz)
GZipped Tar (*.tgz;*.tar.gz)
Microsoft Cabinet (*.cab)
RAR (*.rar)
Rpm archive (*.rpm)
Tar (*.tar)
Zip (*.zip;*.zipx;*.jar;*.ear;*.war;*.bcpkg)
To view the full list of archive formats or change the file extensions associated with them, pick Tools > Options 
and switch to the Archive Types page.
Beyond Compare snapshots
These files are similar to other archives, but only include the file names, dates, and sizes, without including the file content itself. Since they are so small, you can save a snapshot of an entire hard drive and compare against it later.
Total Commander "packer" plug-ins
Beyond Compare can also use Total Commander "packer" plug-ins to support additional archive formats. For example, there are plug-ins to support ISO disc images (.iso) and Microsoft Installer Packages (.msi).
To add one of these third-party plug-ins, on the Archive Types page, click the Add Plug-in button, and follow the instructions.