In the buttons in a palette, you often want to set up a template with placeholders to indicate where expressions should be inserted. \[SelectionPlaceholder] marks the position where an expression that is currently selected should be inserted when the contents of the button are pasted. \[Placeholder] marks other positions where subsequent expressions can be inserted. The Tab key will take you from one such position to the next.
In describing how to enter input into the Wolfram System, it is sometimes useful to give explicit representations for keys you should press. You can do this using characters like ↵ and . Note that ␣ and are actually treated as spacing characters by the Wolfram Language.
The fonts that are distributed with the Wolfram System contain their own renderings of many ordinary keyboard characters. The reason for this is that standard system fonts often do not contain appropriate renderings. For example, ^ and ~ are often drawn small and above the centerline, while for clarity in the Wolfram Language they must be drawn larger and centered on the centerline.