A more thorough explanation:

The automatic-assignment doctrine is a rule in trademark law that states that when an entire business is sold, any common-law trademarks used in that business are automatically transferred to the purchaser without the need for a written assignment, unless there is evidence to the contrary.

For example, if a company sells its entire business to another company, the trademarks associated with that business, such as the company name or logo, would be automatically transferred to the purchasing company.

However, if the trademarks are registered under the Lanham Act or some state registration schemes, a written assignment is required to transfer an interest in a registered mark or in a pending application to register a mark. This means that if a company has a registered trademark, such as the Nike swoosh, a written assignment would be required to transfer ownership of that trademark to another company.

The automatic-assignment doctrine simplifies the process of transferring ownership of trademarks when a business is sold, as it eliminates the need for a separate agreement to transfer ownership of each trademark.