Relationships become clearer when you take advantage of stacking order.
Location isn’t always everything. Sometimes the position of shapes isn't enough to convey a relationship. Additional visual information is required.
For example, take a look at the two clusters of computers shown in this network diagram. You can see that each department has two computers and one printer. The underlying circles tell you so. If those circles weren't there, the situation wouldn’t be so clear.
In Visio, each of those circles is said to be below the computer equipment. And the square is below the two circles. This concept of shapes being under or on top of each other is called stacking order.
You can make complex relationships easy to understand in your diagrams. Read on, and stack up.