Why is Group Id in Audit Trail not filled?

Hi,

Customer is using Audit Trail in his Siebel application. However the Group Id is not filled in the record. He expected, that in this field the row_id of the position or division of the user will be available.

He found out in the bookshelf (Siebel Audit Trail Guide) the following information:

Group Id: "Displays the unique identifier of the group to which the user who changed the record belonged."

Why is "Group Id"  not tracked and not available in any audit trail record?

Thank you