What is a system prompt (or system message) for?

To store the user's private data
To limit the number of tokens the user can send
To set the role, rules and style for the whole conversation, which the model is trained to give more weight than ordinary user text (Correct option)
To make the model learn permanently

To set the role, rules and style for the whole conversation, which the model is trained to give more weight than ordinary user text

The system prompt is where the developer says who the assistant is, what it may and may not do, and how to answer.

It is a strong hint, not a lock. A determined user can still try to override it, so never put secrets in it and never rely on it alone for security.

Real limits belong in code: permissions, validation and filtering.

System prompt = guidance, not a security boundary.