That's the field, which contains a value like "AA2017_258", which is part of the url of the photo. I made a fake one in the image I sent, but the working version of the string in the hyperlink field looks like this: "https://www.nodc.noaa.gov/archive/arc0166/0222391/2.2/data/0-data/LakeMichigan_2017_2018_Videos_Photos/Photos/{Site_ID}_1.png" while the URL uses this for the thumbnail: "https://www.nodc.noaa.gov/archive/arc0166/0222391/2.2/data/0-data/LakeMichigan_2017_2018_Videos_Photos/Photos/thumbnails/{Site_ID}_1.png"
What would be an example of a URL for you and what field would hold the unique part of it?