The Limit Chart to Top N selection criteria allows you to specify how many pie sections should be included in the resulting output.  Including too many sections will render the output unusable and thus this control provides a means to control the output.  

In addition, you can optionally use the Include all remaining items in a single Other category check box, which when checked will combine the results not included in the Top N sections into a single pie slice.  Note that you will not be able to click through the Other pie section.