How do I do that? As seen in http://www.speedrun.com/pmdredblue#Any

Extra variables > check "Use as subcategory"

In order to use the "Use as subcategory" option you need to make at least one option for that variable, and have the checkbox marked to make it a default selection.

Extra info - The variable name is what will be used in the submission menu, the names under it are what will be shown in the dropdown box as options.