I do not have evidence that many people use classes with dashed information flows. In fact, it's even the first time I read this.
Dashed arrows in UML class diagrams have specific meaning, and in particular they show dependencies that exist between classes. This dependencies allow to express that one class creates instances of another for example, or that one operation of one class depends on attributes of another. These dependencies shall not be viewed as information flows.
However, there is a less known UML information flow diagram. These are meant to show at a high level (i.e. not a class with all the details, just the class name), how entities can exchange flow of information. You'll recognise them with the stereotype «flow» that describes the kind of objects that are exchanged. It's mainly used for top-down analysis, i.e. before you arrive at the detail of the real classes.
These information flows have the inconvenience that they cannot to express very well the sequence of events in a complex flow and how different flows a re linked. Only the exchanges between high level objects. Other diagrams are therefore usually preferred, when it comes to model end-to-end flows:
- the communication diagrams (when many classes interact), or sequence diagrams (when a few classes interact). In these diagrams, instances of classes interact with messages.
- the activity diagrams, which allow to show a flow of activities and objects, with a very comprehensive semantics. (Studies have proven that is is also suitable for business process modeling where usually non-UML notations like BPMN or EPC models are used).
