By using the code below, we would be able to use the parent form field value in grid form as a grouping/dependency for a dropdown selectbox.
<script>
$(function(){
    region = window.top.$("input[name='region']:checked").val();
    FormUtil.getField("region").val(region);
});
</script>

With the script above, it will copy the parent form field value from the parent form into the child form. This way, we can now make use of "Field ID to control available options based on Grouping" attribute in the Select Box. This way, we will be able to limit the selections when populating the selections through the JSON Call Form Options Binder in bid to improve performance and narrow down the selections.