Lets you select objects by dragging the mouse over them.

(Paint Selection Region), on the Selection Region flyoutTo change the brush size, right-click the Paint Selection Region button, and then, on the Preference Settings dialog > General tab > Scene Selection group, change the Paint Selection Brush Size value.
If you hold down Ctrl while specifying a region, the affected objects are added to the current selection. Conversely, if you hold down Alt while specifying a region, the affected objects are removed from the current selection.
To select by painting a region:
Choose
(Paint Selection Region) from the flyout.
Drag over the objects to select, then release the mouse button. As you drag, a circle showing the brush radius appears attached to the mouse.
To change the brush size, right-click the Paint Selection Region button, and then, on the Preference Settings dialog > General tab > Scene Selection group, change the Paint Selection Brush Size value.
You can also set keyboard shortcuts for changing the brush size. To do so, use the Paint Selection Size Up and Paint Selection Size Down action items. See Keyboard Panel.