...
This structure represents the unique key fields associated with identifying an accounting period. Accounting periods can be identified by AccountingPeriodCode, AccountingPeriodName or AccountingPeriodUid. When specifying a PwsAccountingPeriodRef as an input to a web service, you may specify any combination of one or all of the AccountingPeriodCode, AccountingPeriodName or AccountingPeriodUidmore of these fields. However, if you do specify more than one key, they must refer to the same accounting period or the request will fail.
...