Left 4 Dead 2
Campaign selection, game modes, plugin frameworks and player slots.
Everything in Your panel applies. These are the Left 4 Dead 2 specifics.
Startup parameters
L4D2 keeps it minimal — the only variable exposed is Map, which sets the campaign the server starts on.
Everything else lives in the server's configuration files, edited in the file manager: game mode (co-op, versus, survival, scavenge), difficulty, slot count and the rest.
Game modes and slots
The default four-player co-op limit is a game rule, not a server limit. Running more than four players in co-op needs a plugin — this is one of the most common things people ask for and it is not a setting we can flip for you.
Versus and scavenge have their own expected player counts, and mixing a mode with the wrong slot count is the usual reason a lobby behaves oddly.
Plugins
Most communities add MetaMod and SourceMod for in-game moderation, votes, kicks and bans — and for the plugins that extend player counts or add custom modes.
They are installed as files through the file manager or SFTP, not from the panel. Restart afterwards.
Custom campaigns
Custom campaigns go into the addons folder. Players need the same campaign installed to join a server running it — unlike Workshop-driven games, there is no automatic download on connect.
Tell people which campaign and where to get it, or expect a lot of failed joins.
Memory
Plans start at 2 GB, which comfortably covers a standard co-op server. Add memory for a large addon collection or a heavily modded setup.
Console
You can change most settings live from the console with the game's own commands, or bake your preferred setup into the config so it applies on every start. The second is better for anything you want permanently.