OpenModelica doesn't really differentiate between class and model, since historically they've been pretty much the same thing. This has been changed in the specification recently though, by restricting class to only be allowed to contain class-definitions, annotations, and extends-clauses. class also has the property that anything can extend a class, while model is really only allowed to extend other models.

I don't think there are many situations where you would actually need to use class now, and the recommendation is to always use the most specialized type of class that you can. In other words, prefer to use model when something is an actual model.

I frequently define icons in classes rather than models. That way I can use the same icon in both models and packages. In Dymola, for example, I get a warning if I extend a package from an icon model. For example:

class valveIcon

  annotation(graphics(...)) // valve icons drawn here;

end valveIcon



model valveModel

  extends valveIcon;

  // equations here

end valveModel



package valvePackage

  extends valveIcon;

  // various valve models inside package

end valvePackage;