In the 2014 manual page 154, it starts a section on adding an equation to drive a part dimension in the assembly.  I personally don't like to recommend this, as there is no 'External Reference' markers in the child part for describing how the dimension is being controlled.  Unless the user of the part knows about the assembly where the equation was made, there is nothing that tells the user about this.  If you double click on the dimension that is driven from the equation you simply get an error indicating that the dimension is read only.

I think introducing feature Comments here would be a decent way to document this relationship.  Attached is the example.  Any thoughts?