In QuickBooks Self-Employed, expense categories line up with the Schedule C tax form. Each time you categorise a transaction, QuickBooks matches it the correct line on your Schedule C form.
Currently, you can't create custom categories in QuickBooks Self-Employed. We're researching how we could approach dynamic categories while maintaining their main use as tax categories for tax forms. We need to do both to ensure your estimated taxes are accurate.
We'll let you know if and when these are available.