It's the general power that CRA has to audit business. As I said, this is similar to other powers the CRA has when it administers other statutes.
The way it's done in practice, CRA doesn't usually just show up at the business and ask questions. Usually, CRA staff contact the person. They may just request information over the phone, or they may ask to visit, to have a better idea of the business, but these are standard practices used by CRA.