Hi @Luca_De_Renzo ,
The definition of one_hot_matrix() function sets depth = 6 as the default value. This means if the one_hot_matrix() is called with no depth is passed in, the default value will be used.
This kind of behaviour is common in Python. There are lots of online resources on Python programming. It would be good if you can familiar yourself with the programming language. Here is a link to Python resources that you may like to start with.
For the unit test, depth is set to 4. The unit test is just to check if your code works. Whoever designed the lab assignment, the main objective is to help you understand the concepts presented in the video lectures.
The mentors here are happy to answer your questions and offer assistance if needed.