This is a relatively new feature. By default, most categories are considered “Expenses”. Default categories cannot be changed. However, if you want to create a custom category, you can do so and specify whether you want it to count as “Income” or as an “Expense”. To do so:

1. Click on “Settings” and then “Categories”
2. Click “Add Category”
3. Enter the name of the new category, and specify whether you want transactions in this category to count as “Income” or “Expenses”