Page 35
"I noticed that almost all of the ballots I reviewed were for Biden. Many batches went 100% for Biden. I also observed that the watermark on at least 3 ballots were solid gray instead of transparent, leading me to believe the ballot was counterfeit. I challenged this and the Elections Director said it was a legitimate ballot and was due to the use of different printers. Many ballots had markings for Biden only, and no markings on the rest of the ballot."
edit: doh! I accidentally her name /shame
I believe that was in reference to checking the ink on the user-filled portion of the ballots in order to see if all of the suspect ones could be batched together. It was said that you could analyze the composition of the ink as a "fingerprint". This was less about "watermark" and more about forensic analysis of the duplicate hand-written ballots.
This might be used to narrow down how many people were in on it and possibly area of origin.