This feature allows users to select multiple objects at once in drawing.tool. In the 2D view, when shift is held, clicking on a non-element area will trigger drag select function. A blue boundary box will show up as cursor moves and it will select all the objects which is inside the box.
When the selection box is drawn from upper-left to bottom-right, it will trigger complete selection which means objects will only be selected when the entirety of the object is inside the selection box.
When the selection box is drawn from upper-right to bottom left, it will trigger the partial selection which means, objects will be selected as long as part of it is inside the selection box.
Happy Modeling!