Objects in Dia can be connected using lines. The Select Transitive and Select Connected options allow you to select objects based on how they are connected.
The Select Transitive option selects ALL objects connected directly or indirectly to the currently selected objects, no matter how many levels deep the connection might be. To use this, select one or more objects and then choose Select->Transitive. All objects connected to the initially selected objects will be added to the selected set.