Professional fees are revenues or expenses of services from people specially trained in specific fields of arts and sciences.
eg as doctors, architects, lawyers, and accountants.
It’s usually the main revenue of firms offering these services (accounting firms, law firms, engineers, etc.).
They are usually in the first part of the income statement of the firms, under revenues. Meanwhile, the businesses that use these services, usually record Professional Fees as an expense, to record costs incurred in employing the services of outside professionals.