System role
Rules, persona, constraints, invariants. Loaded once, applies to the entire conversation. Not user-visible in most UIs. Highest instruction weight in models RLHF'd for hierarchy.
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User role
The actual request. Task-specific. May include user-provided data. This is where prompt injection risk lives.
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Assistant role
Model's outputs. Also used to seed few-shot examples: alternate user/assistant turns to show desired behavior.