Align
Modify > Translate > Align... > Align - This tool requires two objects, one of which should be completely selected, and a click on two polygon normals. The Align tool will move and rotate selected geometry in such a way that the normal of the first click would face the normal of the second click.
To use the Align tool, first activate the tool. Click on a polygon of the selected geometry that you wish to align, then click on the polygon center that represents the target normal on the unselected geometry. All selected geometry will move so that the first normal shares the same root as the target normal, and this geometry will also rotate so that it faces the opposite direction of the target normal.
Note that if you click on the unselected geometry first and then the selected geometry Align will give unexpected results. Always click on the selected geometry first.
An example of Align usage: first click a polygon on the object to align, then click a polygon on an unselected target object. The selected object will move and rotate so that its clicked polygon center faces that of the clicked polygon center on the unselected box. Once aligned, a Transform gizmo permits further adjustment. Right clicking on a different polygon will move the gizmo to another location.