Internal Program Version: squashfs-tools 4.7.5 (The man files)

Squashfs is a highly compressed read-only filesystem for Linux. It uses zlib
compression to compress both files, inodes and directories. Inodes in the
system are very small and all blocks are packed to minimise data overhead.
Block sizes greater than 4K are supported up to a maximum of 64K.
Squashfs is intended for general read-only filesystem use, for archival use
(i.e. in cases where a .tar.gz file may be used), and in constrained block
device/memory systems (e.g. embedded systems) where low overhead is needed.

https://github.com/plougher/squashfs-tools