By default, budgets will display any transaction that:
falls within the budget period,
originates from an account that budget monitors,
is assigned a category that the budget monitors.
Sometimes, however, you might need more fine-tuned control over which transactions arrive in a budget. That's where transaction tags can come in handy. You can create a budget that requires some transaction tags to be present - transactions without those tags would not be displayed in the budget. This allows you to have Personal and Business transactions (tagged accordingly) and separate budgets, while using the same accounts and/or categories.
Please check the How to create a budget? and How to edit a budget? for more information on the transaction tags property in budgets and how they work.