Cross-Reference (XREF) Tables track the base object's data lineage. Each base object has an associated Cross-Reference (XREF) Table that tracks which source system cells contributed to the data in the base object's user-defined columns. For each cell of data in the base object, the XREF table contains:

An identifier indicating which source system contributed the data.
The primary key of the contributing record.
The most recent data provided by the source system.

If more than one source system contributed to the data in a base object cell, the XREF table tracks the information for each contributing source system.