Reading Usable Help
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Gordon R. Meyer
More on embedded Help
Documentation that is integrated into a product's user interface is known as "embedded help" or "UI text." This is often the most effective form of documentation because it isn't abstracted from the user's actions. It literally provides another layer of interface, albeit one that is overtly instructional and (typically) textual in implementation.
It's best when UI text is planned-in from the beginning of a project, but doing so can result in its overuse. If there's too much of it, it clutters the interface and is likely to be ignored.