See Inheritance between classes for an explanation of metadata propagation.
When you open the Propagate Metadata Changes window (described below), it displays all non-system inherited and class level properties (custom properties that are added to this class). It does not display any properties that were previously deleted. If there are no class level properties, then opening the Propagate Metadata Changes window is not in a position to do anything. The intent of letting you open the Propagate Metadata Changes window (as opposed to responding to it when Enterprise Manager automatically pops it up) is to let you propagate the class level properties down to the subclasses later on.
For example: you have edited your class level properties, and when the Enterprise Manager prompts you to propagate your change, you cancel out without doing anything. Later on, you realize that you do want to propagate that change to the subclasses. So you carry out the procedure below and open the Propagate window.
To control the propagation of metadata changes to all subclasses
NOTE Enterprise Manager automatically opens the Propagate Metadata Changes window whenever you do something in which propagation needs to be decided.