A represented buyer is a visitor who already has a buyer’s agent. At an open house they are usually there to see the home, not to find an agent, so they are a courtesy contact rather than a prospect you would try to convert.
Separating represented from unrepresented visitors as they sign in is what turns a flat list into a ranked callback queue. The unrepresented visitors, especially those with a near-term timeline, are the ones to call first.