A dynamic action can have a TRUE action of "add class". The Dynamic Actions sample application (page 7) is an example of this. I haven't found where the available class names are documented. Is there a way to see a list of them?
You can add whatever class name you want, it isn't predefined. This just assigns the particular item the class that you have added.
You might do this if you wanted an item to change property at a given time (very basic example) - when your dynamic action triggers you add the class then you have some css on the page which applies to that class. More complex use cases might involve a JavaScript function which applies to a certain class.