a (possibly lazy) list of completion example strings. These strings are continuations of user's input. The user's input is incremented with calls to withAddedPrefix.
a string that just typed in by the user.
a new source of only those examples that start with the string typed by the user so far (with addition of the just added prefix).
These sources of examples are used in parsers for user input completion. An example of such a source is the sbt.complete.FileExamples class, which provides a list of suggested files to the user as they press the TAB key in the console.