Notes & Comments are very useful, but how and when you use them will vary. Notes are added to the entire diagram whereas comments can be added to individual objects. They can be seen by any named collaborator on the diagram inside Creately. None of them show in published or exported diagrams.
You can add notes in rich text format. They are useful for describing the document or what areas you are editing. They can be edited by any named collaborator on the diagram.
You can use comments to say something about a particular object or a group of objects. When you add a comment to an object and have the "Show comment indicator" ticked a yellow color icon will be display in the corner of that object - as shown in the screenshot below. When you click on a previously added comment inside the Comments tab, the relevant object or or group of objects will be highlighted. Comments can only be deleted by the person who created it and they can't be edited.