Understanding the meaning of [2, 2] for input_register and target_register in a custom Cirq gate

Question

I'm learning a custom gate implementation from google Cirq official tutorial of shor algorithm(https://colab.research.google.com/github/quantumlib/Cirq/blob/master/docs/experiments/shor.ipynb#scrollTo=Q2ZlalhkYArv ), and I'm trying to understand the meaning of the numbers in input_register and target_register. Here's the custom Adder gate:

class Add(cirq.ArithmeticGate):
def __init__(
    self,
    target_register: [int, Sequence[int]],
    input_register: Union[int, Sequence[int]],
):
    self.target_register = target_register
    self.input_register = input_register

def registers(self) -> Sequence[Union[int, Sequence[int]]]:
    return self.target_register, self.input_register

def with_registers(
    self, *new_registers: Union[int, Sequence[int]]
) -> 'Add':
    return Add(*new_registers)

def apply(self, *register_values: int) -> Union[int, Iterable[int]]:
    return sum(register_values)

It defines the input_register and target_register as follows:

adder = Adder(input_register=[2, 2], target_register=[2, 2])

I'd like to understand what each 2 means in input_register = [2, 2] and target_register = [2, 2]. How do these numbers relate to the qubits and their dimensions, or do they have a different meaning? If so, what does it signify?