add discussion of business object life time to Building a system

Table 3.5: Invoice Element

5.X Life Time of Business Objects <After Life cycle of business objects see PTK-194>

Some Business Objects such as Quotation or PurchaseOrder may only be valid for a given period of time. These Business Objects have an @Expires attribute, that defines the date and time when a proposed contract is no longer binding for the sending party unless it is either confirmed or refused. A Business Object that has neither expired nor been confirmed is a "Pending Business Object". A Pending business object can be confirmed by sending the appropriate Business Object as defined in table <##ref business object interactions> that references the pending Business Object. A Pending business object can be explicitly refused by sending a Refusal that references the pending Business Object.