Query: what is the difference between simple and detailed opening. which option should I consider? what is the basis?

how two way flow phenomenon is different in Simple and Detailed opening?

AFN Linkage Node 1 to 2 mass flow rate and AFN Linkage Node 2 to 1 mass flow rate

At times, Why both of these are on (or) giving positive values? (Across windows) what do you mean by that? in that case if I want to determine the net flow into the space Do I need to sum these both flow rates to determine the flow of air into the space.