The second method is to leave the billing plan event in Pending status until the activity associated with the bill event is ready to be billed. For example, you can define a milestone billing plan event and leave it in Pending status.
Then when the milestone is reached, you can update the billing plan event status to Ready to initiate billing. This method enables you to enforce greater control over what bills are generated.
Regardless of the method that you choose, when you set the billing plan event status to Ready, the system performs edit checks to ensure that all required event fields have been defined (Milestone ID, Milestone Number, and Percentage).