Data Transfer Objects are used pass data between two process. They differ from business objects in that they have no methods, they are data only. Because they have no methods, api changes in the processes typically have no impact. Of course adding new data elements to the DTO will be a breaking change.
A webapi or a business layer should return DTO's rather than Business Objects. They should be an abstraction of the underlying data, so processing logic or data store changes have as little impact as possible.