This is the Replace Lookup confirmation window.  As shown below it allows you to confirm the Lookup you are about to replace and choose what to replace it with.

Notes:

1. You must choose a replacement, ie you cannot leave it set as 'None' as shown above.
2. If you check the Delete Lookup? box, then the Lookup you are replacing will be deleted when the replacement is complete. ie in the above case, the Product Group Lookup 'Procedures' would be deleted.