This determines the fallback behavior when a private network's dedicated resources run out; setting it to use-public(1) lets this circuit spill over onto the shared public network's capacity rather than simply failing to establish, trading strict isolation for availability. An admin managing a customer's private network that occasionally needs more capacity than dedicated to it would enable this overflow option, while a customer with strict isolation requirements would leave it unset so their traffic never touches shared public resources even under load. For example, a private-network customer experiencing an unexpected traffic surge with cktPrivateNetOverflow set to use-public(1) would see their circuits keep working by borrowing public network capacity, whereas the same surge with overflow disabled would cause new circuit establishment to fail once private capacity was exhausted.