Hello Youssef
The use of spanning tree protocol (STP) does play a vital role in the three-tier network design model. It plays a role in the particular model layer that is configured to use Layer 2.
In the three-tier model, typically, communication between the core and distribution layers functions at Layer 3, where routing is configured. It would be rare to see an L2 setup between the core and distribution layers, as such an approach would not be scalable. So you almost never see STP operate between the core and distribution layers.
As stated in the lesson, you could have Layer 2 functioning between the distribution and access layers where STP would be employed, such as in the following diagram:
or you could move the operation of Layer 3 all the way down to the access layer switches like so:
In such a case, STP would not play a role. Does that make sense?
I hope this has been helpful!
Laz