AutoGrid used to position the second block on top of the first
When you use AutoGrid, this feature generates and activates a temporary construction plane based on the normal of the face that you initially click. This method is an alternative to creating objects first and then aligning them later.
You can use objects within an XRef scene as reference when AutoGrid is on.
Example: To create a box aligned to another object using a temporary construction grid:
The cursor includes an X,Y,Z tripod to help you orient the position of the new object. As you move over the object, the cursor aligns the Z axis to the surface normal.
When you click, a temporary, automatic grid is created, and the newly created object is aligned to that grid.
AutoGrid is available only after you select an object button (such as Box). When you turn AutoGrid on, the cursor includes an axis tripod to help you orient the grid. Before clicking and as you position the cursor over a visible mesh object, the cursor jumps to the nearest point on that surface. The tripod's X and Y axes form a plane tangent to the object surface (forming an implied construction grid), and the Z axis is perpendicular to the plane.
After creating the object, 3ds Max places it on the temporary construction grid. When creating an object, if the cursor isn't over another object, 3ds Max places the object on the current active grid.