By moving the cursor over existing shapes or the terrain you can snap the position to those objects. An orange rectangle is shown around the cursor when snapping is active. Additionally, the snapped component is highlighted.
Snap to a polygonal edge
After the first click, multiple snapping features are enabled. These include: Global axes, 90 degree angles, parallel lines, extensions of lines and line midpoints. All snapping features are automatically intersected to form combined snapping results. While moving the mouse, an orange dashed line is shown whenever snapping to such a feature occurs. Additionally, symbols are shown to differentiate the features: