For example, if you examine the code in detail here:
https://github.com/ton-blockchain/payment-channels/blob/e605580c3fb1feb22d80be9a0cddfcd05671c347/func/async-channel.func#L583

You will see that the code is the same for both external and internal receive functions:

() recv_internal (int contract_balance, int _, cell _, slice in_msg) {
  return recv_any(contract_balance, in_msg);
}

() recv_external (int contract_balance, int _, cell _, slice in_msg) {
  ;; Note that only cooperative_close and cooperative_commit
  ;; will be accepted
  return recv_any(contract_balance, in_msg);
}

My question is: What is the difference between these functions in FunC, and why does it occur?