This action has one input: `include_prodos`. By setting this to true, it will download the latest release of ProDOS (2.4.2) and extract all of the files to a directory called `PRODOS.2.4.2`. This allows you to use those files, if needed, in subsequent steps. So you can create a bootable disk or even add `BASIC.SYSTEM` to launch into AppleSoft BASIC.
Cadius is a disk image program originally by the amazing French team, Brutal Deluxe. They are not involved with this Open Source version, but I do recommend you view [their site](http://brutaldeluxe.fr/products/crossdevtools/cadius/) for more information on Cadius and their other incredible tools and releases.