Most Tiller templates include a Categories sheet that has a Category, Group, and Type column structure.
You can create a subcategory hierarchy by thinking about Groups as the top level and Categories as the secondary level.
For example, if you want to organize all your food related expenses under a single umbrella, but still be detailed about the specific kind of food purchase you'd use "Food" for the Group and "Groceries, Restaurants, and Snacks" for the Categories.
