Update command is applicable when you need to update all records from your list with values from another list, without adding new records. For example, you've received new prices which should be added to your existing list.
Use the drop-down arrow on the Compare button to select the Update command:
When you select Update command, xlCompare does the following:
You don't get comparison results in this case. This command does all work itself and gives you result of update operation on the left sheet.
So, data from the right book(worksheet) is merged into the data in the left book(worksheet).