What do “Clientside” and “Serverside” mean?
Unlike Arma 3, where you could optionally run certain client-only mods (for example sound tweaks) as long as the server accepted their signatures, Arma Reforger enforces strict mod synchronization. When you join a server, the game automatically downloads and enables exactly the mods that server specifies. Any mods you have locally that are not on the server’s list are disabled for that multiplayer session.
Why do these tags exist on the website, then?
This split is mainly for server admins and mod authors who run servers or assemble mod packs. The tags do not describe who “chooses” to install a mod; they describe where the modification runs technically.
Clientside
These are mods that change things locally on the player’s machine. Typical examples are user interface (UI) changes, inventory tweaks, sound effects, or textures.
What that means for admins: the server admin must still list the mod in the server configuration. Everyone who joins has to download those files, because the work of simulating and drawing the feature happens on each player’s client.
Serverside
These are mods whose scripts and logic run only on the server. Typical examples are admin tools, altered AI behaviour, vehicle spawn systems, or economy-style systems.
What that means for admins: the mod runs on the server hardware. Players often do not need to download much (or anything) when connecting, because their game may look unchanged while the server simply drives different behaviour through the protocol.