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 shows in published or exported diagrams.
You can add notes in rich text format. They are useful for describing the diagram 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 shape or a group of shapes. When you add a comment to a shape and have the "Show comment indicator" ticked a yellow icon will display in the corner of the shape - as shown in the GIF image below. When you click on a previously added comment inside the Comments tab, the relevant shape or group of shapes will be highlighted. Comments can only be deleted by the person who added and they can't be edited.