As you probably know by now, Inkscape’s selection is just a list of objects, which may include anything—from no objects (empty selection, nothing selected) to all the objects in the document. Selection is local to the editing window; if you open a second window with the same document (3.5 Instances, Documents, Views), that window will have its own independent list of selected objects. Selected objects can be anywhere on the canvas, in any layer or group; multiple selected objects don’t have to be children of the same parent. The only thing you cannot do is select an object and its ancestor at the same time (5.10 Selecting in Groups).
Note
Even hidden or locked objects can be included in the selection. While most tools will refuse to directly select a hidden or locked object, it is still possible, for example, by using the XML Editor (4.7 The XML Editor).
On the canvas, each selected object is marked by the selection cue. By default, this cue is a dashed frame around the object, showing that object’s bounding box (4.2 The Bounding Box). This frame is painted on top of all objects using a contrasting color, so it is visible on any background.
You can switch to a different selection cue: a small diamond-shaped mark in the top-left corner of each object’s bounding box, as shown in Figure 5-1 (this is similar to what the Xara vector editor uses). To change the type of the selection cue or to turn it off altogether, go to the Tools ▸ Selector subpage of the Inkscape Preferences dialog.
Also, you can control which tools display the selection cue and which do not. For this, look for checkboxes labeled Show selection cue on each tool’s page in the Inkscape Preferences dialog. By default, every tool shows the selection cue, except for the Calligraphic pen and Paint bucket tools, which are more artistic than technical—there, the cue is not very useful and may be a distraction. (Of course, the selection is still there whether the cue is displayed or not—it remains the same now matter how much you switch tools.)