What is a system message in LLMs?

• A. A log of system diagnostics.
• B. A user profile containing preferences.
• C. A system-level API key.
• D. A message that defines the model's behavior and instructions for structuring responses.

Answer: (D)

System messages set how the model should behave in a conversation. They act as an instruction layer that defines the model’s role, the tone to use, the kinds of content allowed or disallowed, and how to structure responses (for example, whether to be concise, whether to use bullet points, or how to format steps). This is different from user messages, which convey what the user wants, or other elements like internal logs or credentials that aren’t instructions for shaping the reply. The option that describes a message that defines the model’s behavior and instructions for structuring responses captures this role exactly.