When you start to drag Totals into the View, Tableau displays a new menu where you can specify whether you want to add the subtotals or the grand totals for Rows or Columns. Drop the option on the desired box to add Totals. Tableau uses the default property Total using of the Measure to choose the aggregation. 

Here's an example of subtotals and grand totals:

Now, let's explore a new set of options, Model.