This property is accessed to query the child key column name. Combined keys can be passed with the separator "\t". Your data provider should therefore deliver for example "CustomerID\tOrderID" and the corresponding child fields. In the ApplyFilter() Method of the ITable Interface the values are arriving exactly like that, a filter string could look like following:
"CustomerID\tOrderID=22\t14"
The GetChildTable() Method of the ITableRow Interface directly gets a ITableRelation Interface, so that you also have access to the combined fields there.
Platforms: Windows 10 (Version 21H2 - 23H2), Windows 11 (21H2 - 22H2), Windows Server 2016 - 2022
.NET: .NET Framework 4.8, .NET 6, .NET 8, .NET 9