It’s extra. The term is typically associated with budgets, whether a business or government. If the government spends less than it brings in with tax revenue and fees, it runs a surplus (instead of a defective when it spends more than it takes in). A business department gets an annual budget to spend for salaries, equipment, supplies, vendors, etc and if they don’t spend it all then they have a budget surplus. You’ll often see people responsible for a budget avoid a surplus because it is likely to lead to a smaller budget in subsequent years.