Links are written in the ALE and IDoc environment which result in entries in the IDOCREL and SRRELROLES tables. These links are required for IDoc monitoring for the document trace and for the ALE audit.
The business objects in a link are stored in the SRRELROLES table. The business objects are assigned a role type which specifies the role played by the business objects in the links.
The following link model is based on the links in the IDOCREL table:

All business objects can occur in the INBELEG and OUTBELEG role. The INIDOC and OUTIDOC roles are reserved for the IDoc object type and the INTID and OUTID roles are reserved for the TRANSID object type.

• Document trace

• IDC0 links the outbound document and the outbound IDoc

• IDC1 links the inbound document and the inbound IDoc

• IDCB links the inbound IDoc and the outbound document. This link is written due to certain settings in the IDoc Customizing for the message types used.

• ALE audit

• IDC4 is provided for the link between the outbound IDoc and the inbound IDoc. The link is written in the receiving system from the inbound IDoc to the outbound IDoc if the sending system is a logical system ('LS'). If an ALE audit was executed in the receiving system, an additional link is written from the outbound IDoc to the inbound IDoc in the original IDoc sending system.

• Monitoring

• IDC8 links the inbound IDoc with the tRFC ID in the inbound IDoc. This link is used for everything from troubleshooting to the complete IDoc update.

• IDCA links the outbound IDoc with the tRFC ID in the outbound IDoc.

You can use the RSRLDREL report to reorganize the entries in the IDOCREL and SRRELROLES tables. Some recommendations are provided below:

• You can delete IDC8 and IDCA links on a regular basis. The links are generally no longer required after the IDocs are posted successfully in the target system.

• Select RSRLDREL under 'Selection mode':'Selection via link type' on the selection screen of the report and create a variant for both the IDC8 and IDCA link types.

• Select 'Deletion criteria': 'Without Existence Check'.

• When you delete the IDC0, IDC1, IDC4 and IDCB links, information that may be important for a subsequent document trace disappears. You can delete these links if the linked business objects do not require a document trace.

If you archive documents and IDocs on a regular basis, you have the option (as of Release 6.20) to archive the link information with the IDocs. You can delete these links in this case.

• If you only want to delete links to certain business objects, open the selection screen of the RSRLDREL report under 'Selection mode': 'Selection via object/role', enter the requested object types and the INBELEG role for inbound documents or OUTBELEG for outbound documents.

• If you want to delete all links with IDocs, enter IDOC as the object type and do not enter anything into the input field for the role. If not all links are deleted, continue by specifying the object type and the role.

• If you only want to delete links for which one or both objects have already been archived and deleted in the system, you can select this in the 'Deletion criteria' section.

Nothing is deleted in test mode. For all link categories that correspond to the selection criteria, you receive the number of database entries in the IDOCREL table as well as the role entries in the SRRELROLES table.

• Note 384660 describes how a modification can stop the IDC0, IDC1, and IDCB links being written.

• Note 574349 describes how writing IDC4 links can be stopped in cases where IDocs are sent from an external system.

Refer also to related notes on the RSRLDREL report and the performance of the link service.
If you are unsure about which documents are hidden behind an object type, you can check this via transaction SWO1.