When you drop a grid on a form and go through the wizard there are a number of places where "Default Behavior" is an option (for example, under Features -> Updating -> Allow Row Adding). Where do you set these default behaviours? We tend to have far more read only grids than updateable ones, so if I could specify this default behaviour somewhere for an entire application it would save having to remember to do it individually.
A 'Default' value in the context of an override object means that it is going to default to the value defined higher up in the heirarchy (column -> band -> grid). If the top level object is also set to 'Default' it is going to resolve to the default behavior as intended by Infragistics; this is not user definable.
In the specific case of AllowAddNew, the "default" behavior depends on the context. If the grid is not attached to a data source it will be No, otherwise it will be Yes.