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Transaction categories are one of the transaction properties that you can set. There is a separate list of categories for expenses and a separate list for incomes. On first use, MoneyWiz creates a default list of categories for you but you can easily create new ones, modify existing or delete the ones you don't need.
If you go to two shops - let's say Tesco and Walmart - to buy groceries, you will have different payees for each of those transactions but the same category: groceries. This allows you to generate category-based reports to see how much you're spending on groceries overall, regardless of the shop you're buying in (of course, it is possible to customize the report to filter some things out).