As we can see on page 630-634 and page 655, the orb’s offerings consist of 6 ‘clips’ or ‘visions’.
The first two shown are important historical landmarks in the real world:
(1) The landing of Viking 1 on Mars on July 20, 1976.
(2) Seminary Ridge and Cemetary Ridge at the time just around the Battle of Gettysburg in the Civil War.
(3) The story referenced earlier in the text with the two cave-children who die.
(4) Clip #I: A girl named Audra Toland drowns (Cas searches for and finds the body later)
(5) A baby named Alvin Alex Anderson lies in his crib in Wichita, Kansas.
(6) Clip #6: Xanther running (possibly to rescue the cat, but probably not, because in the orb, she is smiling.)
As Cas mentions on pg. 636 The first orb visions were “historically cross-referenced”, which suggests to me that the orb orders appear in the book the same order that they appear to the characters. So why name the girl’s drowning Clip I? Because it was the first vision they could definitively prove?