Mod packs are third-party packages that add additional functionality to servers. Users can choose a mod pack they want to user with their server on the the Mod Packs page.
The mod pack list on the Edit Server page shows all mod packs available to that server. Initially when a new server is created, the mod pack list consists of mod packs added by admins to the slave the server is hosted on. The list is unique to each slave, and if you add a mod pack to one slave, it will be available on servers hosted on that slave only.
Users can add their own mod packs to list by uploading them to the root directory of their server. This allows them to use a custom mod pack with their server.
To add a new mod pack for servers on a slave, simply place the file to the
directory on the slave. You can do this using FTP, SCP, wget, curl or any other method you want.