You can check the box for "Add custom categories to this test?" in the "Test options" section of the Custom Test Builder (or when editing a custom test). This allows you to assign each question on the test to a different category.
If you've enabled custom categories, then the test reports will not only show overall scores but also include a breakdown of each category. For example, you might have a test that covers a wide range of topics. Adding categories will allow you to see at a glance how the applicant performed on math, grammar, spelling, etc.
Once you've checked the box, you'll see a new "Categories" section beneath "Test Options." You can add as many categories as you like.
When you add or edit questions to this test, you'll now be able to choose a category from the Category drop-down menu. If you want to add, edit, or remove categories from the test, you'll need to edit the overall test.
For more context, see our complete walk-through of the Custom Test Builder.