|Syntax and mark-up / Working with mark-up|
The class attribute is used in specialisation, and should not be set by a DITA author. The class attribute should not be confused with the outputclass attribute.
All DITA elements have a class attribute, but as a DITA author, you should never change this attribute. In fact, most DITA editors will hide it from you. The DITA class attribute is not the same as the HTML class attribute. (DITA's outputclass attribute is the closest equivalent to HTML class.)
All DITA based processors use the class attribute to determine how an element should be processed, rather than by using the element name itself. This is because specialisation may make the element name unfamiliar to the processor, but the class will indicate the element's inheritance, and therefore how it should be handled.