How are open sale orders, with future dates, taken into account?

If the total quantity of future dated sales orders is larger than the system's forecast for that month, then that takes precedence. Otherwise the system's forecast is used. 

Usually (ie: if the future sales orders are less than the systems forecast), these are consistent with your overall demand trends, so there is no need to explicitly take them into account, since the forecast will automatically cover the expected sale. This will be the "Forecasted Demand" calculated field.

Another option is to manually override the sales forecasts in each month - In the demand analysis set the "Set Manually" amounts for each future month. For manufactured products, this will then update the forecast for each of their components. You can also do this in bulk via CSV, under Load Data -> Show bulk forecast upload.

It's worth noting that it's always unclear whether a future sale order is in addition to the predicted demand, or whether it's just part of the business-as-usual demand and already taken into account from the sales-history-based forecast.

So that's why the system is designed with this extra option to manually set the demand, so that it's clear for both you and when the future calculations are made.