That's
printed side tabs in the outer margins, particularly tabs that bleed to the page edge to provide a clue to readers looking for something without even opening it up.
Not exact matches
The corner where the two
sides of the column meet (and the long glued
tab is visible on the outside of the column) should be positioned at the corner indicated by the dot on the
printed square.
For each slope, fold the two long edges and the shaded
tab upward, leaving the
printed square hidden on the bottom
side.