Hi Federico
Mode-C is very difficult to handle, each time a new Mode-C response is received, a machine learning algorithm attempts to determine if this is a previously received response. Bear in mind the only data we have available for a Mode-C response is an altitude, a signal strength and a time of arrival
This data is combined to see how likely this is to an existing aircraft in our database, terms of its delta change of alt/strength/time, this algorithm is also used to determine that it is not an alias to an existing Mode-S or ADS-B transponder response.
If the machine learning algorithm determines it is a new aircraft, a counter is incremented and this new value is assigned to this response, and prefixed with C-, so the figure you see is a hexadecimal counter which goes from C-0001 to C-FFFF
That is a very simple explanation of an algorithm which is incredibly complicated
Thx
Lee