Figuring out defined groups in a mesh
 

Hi, I found a youtube tutotial about creating a fire simulation model. But, confused about the mesh.

In the fig 1, you will notice that the base and inlet is in the same plane. How is it defined in the mesh? Please explain (look into 2nd img as well)

In the fig 2, you will see that there is another group named "frontAndBackplanes" which part of the mesh is defined by it?

Would really appreciate a reply!

1)https://ibb.co/kyKnvW4
2)https://ibb.co/pKRp898