Returns a value that indicates the way the header source is being supplied for the mail merge operation. Can be one of the following WdMailMergeDataSource constants: wdMergeInfoFromAccessDDE, wdMergeInfoFromExcelDDE, wdMergeInfoFromMSQueryDDE, wdMergeInfoFromODBC, wdMergeInfoFromWord, or wdNoMergeInfo. Read-only Long.