What virtual column does hive.exec.rowoffset add?

[Edward Capriolo]

<property>
  <name>hive.exec.rowoffset</name>
  <value>false</value>
  <description>Whether to provide the row offset virtual column</description>
</property>

I know the others are INPUT__FILE__NAME and BLOCK_OFFSET.... Does
anyone know what this third columns is?

Edward