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 - **Type** — The server software flavor: `VANILLA`, `FORGE`, or `FABRIC`. Determines which mod loader is installed. Chosen during Instance creation and cannot be changed afterward (changing Type means creating a new Instance).
 
+- **Mod** — A Forge or Fabric mod packaged as a `.jar` file (or nested inside a `.zip` archive) that extends the Minecraft server with new features. Uploaded per-Instance into `data/mods/`.
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+- **Mod Validation** — The two-layer inspection applied to every uploaded Mod before it is written to disk: (1) metadata check — reads `META-INF/mods.toml` (Forge) or `fabric.mod.json` (Fabric) for the declared side; (2) class scan — walks all `.class` entries in the jar looking for references to `net/minecraft/server` vs `net/minecraft/client`. A Mod is **accepted** if it contains any server-class reference (with or without client classes). It is **rejected** if every class references only the client side.
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+- **Client-side-only Mod** — A Mod that contains zero server-side classes. These are rejected because they would crash the Minecraft server with a `ClassNotFoundException` or linkage error.
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 - **Modpack** — An optional set of server-side mods packaged as a ZIP. Applied on top of a World + Version + Type to produce a modded Server Instance.
 
 - **Backup** — A full snapshot of an Instance's entire `/data` directory at a point in time (world, server properties, mods, whitelist — everything). Stored as a ZIP archive inside the Server Directory. Restoring a Backup creates a new Server Instance; it does not overwrite the original.