Minecraft Java is the game you join from. Horizon and Pumpkin are independent server projects written in Rust, not official Minecraft editions.
Compare the engines
Horizon
Horizon rewrites Minecraft Java in Rust and tests its behaviour against vanilla. Choose Horizon (Beta) in Manage to try it with the standard Java 26.3 client.
Horizon on GitHub ↗Documentation ↗
Pumpkin
Pumpkin is a separate Minecraft server implementation with its own configuration and plugin ecosystem. The ChunkArc beta runs it with Java 26.3 clients.
Pumpkin on GitHub ↗Documentation ↗
How to create a Rust server
Open Manage and select Create server → Minecraft Rust Edition. Choose Horizon or Pumpkin and your region. ChunkArc sets up the server automatically; you do not need to upload files. When it is ready, copy the address into Multiplayer in Minecraft Java 26.3.
Both engines are experimental. Manage does not offer its mod and plugin catalogue for these servers. Game behaviour can differ from vanilla; keep a copy of any world you want to preserve.