The extra credit (EC) feature in Gradebook can be enabled (1) at the item level or (2) at the category level.  For more information on adding items/categories to the gradebook see How do I set up my Gradebook? or How do I add items to the Gradebook?

When you designate an item or a category as EC, those items are not added to the total "out of" value for points possible. If students earn points for extra credit items, those points are added on top of the total grade. However, no points will be deducted for students who do not receive a score for extra credit. EC indicates "bonus" items, or optional credit.

Note: It is important that you DO NOT make individual items extra credit within an extra credit category. Those items will be considered optional within the category and therefore would have no effect on the overall grade outside of the category.