If you select the whole paragraph and pick a class in the css class dropdown, the class of your paragraph will be changed.

 

If you only select a piece of text instead of the whole paragraph,  CE will use the span tag to apply the css class.

 

CE also has another method which allows you change the class of the existing element.

 

Put your mouse cursor over the element, right click, in the context menu, choose tags, in the open style builder, you can change the class of this element.