You will see white and colored paper that was glued to the bone pieces as identification tags. These were left on the ilium when the 3D photogrammetry was done but they were later removed once the database identifier was applied.
One piece of the ilium was originally attached to vertebra 9 but was removed to assemble the ilium in prep. When you look at the 3D of vertebra 9 you will see it is still there, attached, in 3D space. This makes it easier to "assemble" the specimen in virtual space. It is a proven attachment point.
Dinosaur bones are called "cervical" for the neck; "dorsal" for the main body; "sacral" for the area in between the hip bones (iliums) and "caudal" for the tail bones. The bones were numbered in "prep order" because you don't actually know what you have until all the prep is done.
Because we know for sure vertebra 9 was attached to the ilium, we know vertebra 8 is technically caudal vert 1.
The 3D model and still .jpg measurement photos for this fossil can be accessed from Zenodo: https://zenodo.org/uploads/13261771
Here is a comparison of sauropod pelvis areas drawn by Russell Hawley, including a haplocanthosaurus: