a primary reference that supplies information
The SBAC will ask you to evaluate the sources they provide, which means you must read and analyze what the reference is and evaluate the type of information it provides. What type of source is it? Is it likely to be a credible source? The sources they refer to will be informational texts of some sort, but you will need to be able to identify the main idea or claim made in each and evaluate its effectiveness in terms of support and reliability.