Gamemodes
A gamemode is the single mod designated as a session’s baseline ruleset — free-roam, deathmatch, hide & seek, and so on. Mechanically it is just a mod; the designation is a label. The default is a bundled free-roam gamemode, so a bare server is playable with no setup.
A gamemode owns session rules: who spawns, when, and where; rounds and phases; win conditions; teams. Other mods layer on top and can override even the gamemode’s behavior.