These files are for the user-created Codexes below:
To use these files, first download the relevant 40k data set within Battlescribe. You may need to select "Show Archived Repositories" in order to see the 8th edition data set.
Next, download the .cat file(s) you want from here.
You then need to manually copy the file into your Battlescribe data directory.
On Windows, that is "%userprofile%\Battlescribe\data\Warhammer 40,000 (Nth) Edition" by default.
On Android, it is "\Android\data\net.battlescribe.mobile.rostereditor\files\data\Warhammer 40,000 (Nth) Edition". If you have Android 11 or later, you will need a file manager app that can access system folders.
Once the file is in that directory, you should see the new catalogue as a choice when you go to create a new Roster.