`SET_USER` updates the current session’s user identifier. This is useful when a dialog authenticates a user and wants to associate subsequent interactions with that user’s record.
**Example (from `auth.bas`)**
```basic
SET_USER "550e8400-e29b-41d4-a716-446655440000"
TALK "User authenticated."
```
After execution, all future bot messages in this session are linked to the specified user ID.