This simply means that there are no categories inside this account and hence a default category is selected whenever you categorize a transaction using this account.

For example, inside the Income > Sales, if you haven't mentioned any category, Open keeps a default Sales category to which all your invoices will be categorized. However, if you want to create a separate category for your hardware sales and software sales, you can create these two categories inside sales account. Next time when you create and invoice or mark category for sales, you’d be able to choose the hardware/software/default to categorise your income at a deeper level.

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