All Zigbee devices will use the “cheapest” route to the controller and the controller can handle 20 (aprox?) directly connected Zigbee nodes. When all direct connections are used the devices will start to route through other devices (those that are marked as routers).
The device list shows the current route for a device, Zigbee devices hold a list of other nodes in their routingtable and can change route if needed, e.g. when a routing unit is remover or if the device are moved.
A unit can be routed through another device before 20 devices has been reached, this is normally because of the “cost” for the route is better than communicating directly. This can be because of the distance or radio interference.
It is always a good idea to start including routing devices before adding end devices (normally battery operated units are end devices).