Well from an audit perspective there are several scenarios but the trigger in Netsuite ARM is booking of revenue (through the month end JE run) which has yet to be billed - with no invoice a counter JE booking for unbilled receivables is created. Now note that this may happen way later if a mistake was made, it can also get stuck depending on your setting for allowing changes on sales transactions (if change is to be mirrored on the revenue arrangement or not)
Did you have a specific issue?