Blue Flag Rule.

If you are being lapped by faster traffic and are not in jeopardy of giving up a position or giving up an overtake opportunity, it is your responsibility to allow for the lapping traffic to pass safely.

Communication is key. It is as simple as letting the passing driver know where to make the pass. The time from which lead traffic makes the lapped traffic aware of their presence to the time of the actual submission for pass should not be greater than a single lap. All passing and avoidable contact rules still apply here. This means that the responsibility of safely overtaking the slower traffic falls on the faster traffic, while it is up to the lapped traffic to provide for the overtake in reasonable time.

If you are found in violation of Blue Flag rules, the series admin will have the final say in proper punishment.

Final note:
This is part of proper racing etiquette.