Notes about what you don't know (open questions), empty outline slots, red links as [[wikilinks]], and other "holes" in tools for thought provide a bookmark for where one may have quit exploring, but are an explicit breadcrumb for picking up that line of thought and continuing it at a future time.

Linear writing in one's notebooks, books they're reading, and other places doesn't always provide an explicit space which invites the reader or writer to fill them in. One has to train themselves to annotate in the margins to have a conversation with the text. Until one sees these empty spaces as inviting spaces they can be invisible to the eye.