By default Tiller spreadsheets include a Categories sheet that has a Category, Group, and Type column structure.
The hierarchy for these columns is:
Type (Income, Expense, Transfer)
If you're used to a Category > Subcategory structure think about Groups as the top level and Categories as the secondary level.
For example, if you want to organize all your food related expenses in a single bucket, 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.
To prevent breaking other dashboards/visualizations, do not rename columns in the Categories or Transactions sheet.