Document Index for Retrospective Business Volume Update 
Use
In order to carry out the retrospective update of business volumes, the system must be able to efficiently access all documents that may be relevant to a retrospectively entered condition record. This is done using the "document index", which is an information structure within the Logistics Information System (structure S111), and is also used by other applications in the SAP System.
The system supplies the information structure with the necessary data when a document item relevant to subsequent settlement is created, changed or deleted. For each document item, the function creates an entry containing the document number, the item number and the price determination date, each time the access sequence is accessed.
Prerequisites
So that the system can recognize document items that are relevant for subsequent settlement, you must set the following indicators for the vendor in the purchasing view in the vendor master:
The system uses this indicator to see which document items are relevant for subsequent settlement. Business volume data is only automatically updated for these document items.
Set this indicator for each supplier of the goods listed in the purchasing document, so that the system can create the relevant index entries. The actual supplier of the goods can differ from the settlement partner. In view of the high volume of data, however, you should carefully consider whether you need to set this indicator.
In the case of settlement requests, the system creates customer-side index entries if the relevant indicator is set for the vendor. In the case of documents without a purchasing organization, the system always creates the index entries.

If you set the Subsequent settlement index indicator retrospectively, for example when putting the mechanism into operation for the first time, or if an access sequence is added or changed, the corresponding index entries may be missing in documents that already exist. In this case, you must recompile the document index.
If you also set the Subsequent settlement indicator for the first time when you set the Subsequent settlement index indicator, it is possible that all existing document items are flagged as not relevant to the subsequent settlement process. A requirement in the calculation schema prevents conditions involving subsequent settlement from being inserted for certain document items (e.g. delivery without charge). Similarly, the system does not insert such document items in the document index. Select the indicator and retrospectively compile the document index.
For further information, see

Any contract items that exist should be flagged as relevant for subsequent settlement. Otherwise, newly created PO items referencing an outline agreement item will similarly not be relevant to the subsequent settlement process. You may have to subsequently maintain existing purchase order items that reference the contract.
You should therefore pick out all affected documents (purchase orders, delivery schedules, and outline purchase agreements) relating to the goods supplier and set the Settlement indicator in the "additional data" of the relevant document items (Overview
® Goto ® Further data ® Additional data).