O'Reilly logo
  • Robert Richards thinks this is interesting:

and the key value for the clustered index, ContactTypeId. This is possible because the Lookup operator for non-clustered indexes to a clustered index is the key value of the clustered index.

From

Cover of SQL Server Execution Plans, 2nd Edition

Note

I don't fully understand this info - so - a non-clustered index always has lookup key value?