Canvas is a workspace for writing and code that need further attention. You can edit directly, select a passage and ask for help with that particular part. It suits work already in progress when you want to refine the wording without starting the whole piece again.
Imagine writing an invitation to a photography evening. Start with a simple draft. Then ask for a warmer opening, shorter practical information or an ending that makes it easy to say yes.
One useful instruction might be: “Keep my tone and all the details. Only suggest changes that make the text clearer.” That gives you a clear direction when reviewing the suggestions.
Finally, read the text aloud to yourself. Does it sound like something you would say? Keep the phrases you like and change the rest. AI can help with the craft, while you decide what the writing should convey.