On Fold3.com we arrange all of our documents by collection, and then by increasingly specific categories, down to the individual document level. This structure is visible to you in Browse, and in the Viewer filmstrip. Both of these are available from the viewer page. You will notice that any time you are viewing an image in the Viewer, toward the top of the page you will see a "breadcrumb" of the categories that the document is in. For example, in this image you will see a breadcrumb that tells you that the image is found in Photos > Vietnam War - Army > Boats > Barges, and is named Page 1.
Each of those categories in the breadcrumb can be clicked to show the entire browse tool that you can find on the regular Browse page, but without leaving the Viewer. By scrolling and clicking on different categories in browse, you can access any other image on our site.
Understanding the browse structure is also helpful when using the Viewer filmstrip, which shows the image you are viewer along with neighboring images, arranged according to the same browse structure.