This mode permits selecting all features intersecting the specified rectangle (that is, having common points with it). The system opens an auxiliary window “Selection rectangle”.
To specify selection rectangle it is necessary to place the cursor in one of the corners of the rectangle, push the left mouse button, move the cursor to the opposite corner of the rectangle and release the button.
The set selection threshold is used to determine selected features.
Additional possibilities of performing selection and deselection of features are connected with using the keys Ctrl, Alt, Shift.
The button 
of the mode toolbar in map browser window.
Modes of selecting by rectangle, circle, polyline or polygon
Using the keys Ctrl, Alt, Shift when performing selection
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