What is that icon on top of note page


when you clic the note application in the left menu, above the notepads icons, at the left there are 3 icons:
the 1st one (a watch) for recent notes,
the middle one (notepad with pencil) is to display all the notepads,
but the 3rd one (notepad with lines and pencil), what is it used for?

when i clic it the list is empty.