Authoritativeness means primarily « reputation ». With this perspective in mind, the Quality Raters should consider the entire website and the particular author of the content.
« Look for reviews, references, recommendations by experts, news articles, and other credible information created/written by individuals about the website […] Look for information written by a person, not statistics or other machine-compiled information. News articles, Wikipedia articles, blog posts, magazine articles, forum discussions, and ratings from independent organizations can all be sources of reputation information. Look for independent, credible sources of information. »
The more independent voices are added up, the more an author is seen as an expert on other sites. The more positive reviews for good experiences with a company are reported, the higher the level of expertise is considered.
As in link building, the relevant context is decisive here. Expertise must be proven for those topics that a website should be listed in the Google search results.