Main toolbar > Rectangular Select Region (Selection Region flyout)
The Rectangular Selection Region button, available from the Selection Region flyout, provides one of three methods you can use to select objects by region. The other two methods are Circular and Fence.
For all three methods, you can select either objects that are completely within the selection region (window method), or objects that are either within or touched by the selection shape (crossing method). Toggle between the window and crossing selection methods by using the Crossing Selection button on the status bar.
Procedure
To select using a rectangle:
Click the Rectangular Selection Region button.
Drag in a viewport, then release the mouse. The first location you click is one corner of the rectangle, and where you release the mouse is the opposite corner.
To cancel the selection, right-click before you release the mouse.