You have a long draft and want to rework the third paragraph while leaving the rest alone, over several rounds. What is the right surface for that?

The principle — once a piece of work has to be revised in place rather than regenerated, you need a document rather than a conversation.

Why the key is correct — OpenAI describes canvas as a new interface for working with ChatGPT on writing and coding projects that require editing and revisions, and states that you can highlight specific sections to indicate exactly what you want ChatGPT to focus on. That is your requirement stated almost word for word: one paragraph changes, the rest stays.

Why the others are wrong — a project decides where material lives rather than how you revise it, so you would still be pasting paragraphs into chat. Repeated messages leave the draft scattered across the conversation for you to reassemble. And study mode explains why something is weak, which is a different activity from changing it.

Remember this — two things make canvas worth reaching for beyond the highlighting: you can edit the text directly yourself, and you can restore previous versions using the back button, so an experiment that goes wrong costs you nothing.

Sources — OpenAI's canvas article and Capabilities Overview.