After you've spent some time categorizing your photos, you may want to find ones that you havne't categorized yet. To filter only photos that are missing a particular kind of category, choose Filter ▶ Photos without Date Category, etc. Because most photos have a person and/or a thing, these kinds of categories have been combined into a single filter.

If you've spent time naming your photos, Shoebox can help you assign them to categories. Click a category in the outline, and choose Categories ▶ Show Related Photos. Shoebox will search for all photos whose file name contains the category's name, and which aren't in the category. You can then choose which photos to add to the category.