The fields that appear on the left side of the join belong to the source table.
[Visual Basic .NET] Public ReadOnly Property SourceTable As ITable
[C#] public ITable SourceTable {get;}
[C++]
HRESULT get_SourceTable(
ITable** SourceTable
);
[C++]Parameters
SourceTable [out, retval]SourceTable is a parameter of type ITable
Product Availability
Available with ArcGIS Engine, ArcGIS Desktop, and ArcGIS Server.