Cloning Tails from installer copies all files from the expected directories in the live partition
With options Clone & Install and Clone & Upgrade the Tails Installer copies not only the necessary live system files but also all other files in the expected top level directories on the same partition.
So if tails is installed on a USB stick or a SD card and the user stored some other files on the same partition, in any of these expected directories, those files will be copied too.
The top level directories that are copied are
liveos_toplevel_files in https://gitlab.tails.boum.org/tails/installer/-/blob/master/tails_installer/config.py.