Surface law Carrez: What is it?

Need to calculate a surface law Carrez? Want to learn a little more about how it is established?
Here is a diagram to understand just what it takes into account.
The private area, called Carrez area, is the floor area of enclosed and covered spaces after deduction of the areas occupied by walls, partitions, steps and staircases, ducts, door and window embrasures. Floor areas of rooms with a height of less than 1.80 m3 are not taken into account.
via Condominium Audit & Diagnostics