Sometimes the placement of objects is only considered when defining the area group constraints. This can leave routing without the ability to form all its connections. The following image shows a variety of area group constraints that all allow routing to form connections. In all three of these cases the routing never has to leave the defined area groups to complete its routing.
In contrast there are two common errors with routing and area group constraints. The left side of the following image shows a missing area group for an object. The second case is one where all objects are contained within separate area groups but the two groups are not adjacent. In this case the router has no way to complete the routing of its nets without violating an area group constraint and the router will fail to find a legal solution.