6.10 What is meant by Pre-Depends?

"Pre-Depends" is a special dependency. In the case of most packages, dpkg will unpack its archive file (i.e., its .deb file) independently of whether or not the files on which it depends exist on the system. Simplistically, unpacking means that dpkg will extract the files from the archive file that were meant to be installed on your file system, and put them in place. If those packages depend on the existence of some other packages on your system, dpkg will refuse to complete the installation (by executing its "configure" action) until the other packages are installed.

However, for some packages, dpkg will refuse even to unpack them until certain dependencies are resolved. Such packages are said to "Pre-depend" on the presence of some other packages. The Debian project provided this mechanism to support the safe upgrading of systems from a.out format to ELF format, where the order in which packages were unpacked was critical. There are other large upgrade situations where this method is useful, e.g. the packages with the required priority and their LibC dependency.

As before, more detailed information about this can be found in the Policy manual.