Not all variables have to be defined by the user and associated with render extension and action properties. Storyboard defines a set of variables for the model element properties such as position, size, and visibility. Each class of model elements contains a slightly different set of values, but all of the variables are prefixed with the grd_ (Graphical Runtime Data) variable namespace to avoid any confusion with user created variables.

These variables are generally accessed using ${model_object:varname}, for example ${control:grd_x} indicates the x position of the current control