The two sort buttons on the toolbar and in tooltips manually sort a selection either in ascending or descending order based on the other fields in the view. For example, the view below shows sales by product category and customer segment. When you select the Corporate Market column, thus selecting all of the products in that segment, the quick sort buttons sorts the product field by SUM(Sales), which is the measure in the view.
An easy way to anticipate how a selection will be sorted is to make a selection in the view and hover over the ascending or descending sort buttons on the toolbar. The tooltip for each button describes how the selection will be sorted.
Using the quick sort buttons on the toolbar or in the tooltip creates a manual sort which you can always modify using the sort dialog box. Right-click a sorted field (indicated with bold text) and select Sort to open the Sort dialog box.