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Node-RED has four types of variables: message variables that travel with msg through your nodes, and context variables in three scopes—node (private to a single function node), flow (shared within one tab), and global (shared across the entire instance)—plus environment variables for configuration and secrets. You set and retrieve context variables with global.set()/global.get(), flow.set()/flow.get(), and context.set()/context.get() in function nodes, or visually through the change node. Context variables are stored in memory by default and are lost on restart, so any state that must survive a reboot needs persistent storage.