It is sometimes useful to obscure some amount of a form from export. For instance, if you have a series of hotspots that allow the user to change pen drawing colors, these are useful during a user's use of a form, but might not look appropriate on a rendered PDF. The attached example does precisely that.
In particular, take a look at the datapaths that are configured and notice that "Regions" setting of each PDF is set to "Render". Then look at the script code behind the form. The region's visibility (that is its ability to obscure elements of the form underneath it) is toggled appropriately for each export.