A counter is a series circuit that converts the pulses received at the input into a code proportional to their number.
The main static parameter of the counter is the counting module, which characterizes the maximum number of pulses, after which the counter is set to its initial state.
Consider the count sequence:
Thus, we can say that this counter counts from 0101 to 1110, and then returns to 0101. So it goes through only 10 different states - thus, it is a MOD-10 counter (counter module is 10).