Difference between setWidget and addWidget

Is there a practical difference between the methods setWidget() and addWidget()? Or it depends only on the object which they belong?

Thank you :)

Need more information on which class's 'setWidget()' and 'addWidget()' you are talking about.

I would like to know if there are differences without considering the classes. I will be more specific soon :)

Maybe English is not your first language, but "set" usually means you are replacing one widget instance (which could be null) with another one. "Add" usually means that you are putting one or more widgets into a list that could be arbitrarily long.