Reimbursement of medical bills - basis of calculation

Hi all,
 
I have a question on the Belgian healthcare system. 

Whenever you visit a doctor, you have to pay a certain amount for the doctor's services and he/she will then give you this green paper slip. You then submit the paper to your "mutualité" which will reimburse you a certain percentage of the amount you paid, but without ever explaining how they came to exaclty that percentage.
 
What I don't understand here and haven't been able to figure out by researching myself is
 
(1) on what basis does the doctor determine the fee amount? (usually they scribble something illegible or sometimes just a dash on the green paper slip)
(2) on what basis does the mutualité calculate what you get back? Is there actually a legal basis like a regulation? If so, where can I find it?
 
Thanks a lot for your answers!!
 
Manuel