var ITEM = (label.fields.field.(@name == 'ITEM_NUMBER').@data);
var CPN = (label.fields.field.(@name == 'CUSTOMER_ITEM_NUMBER').@data);
var CIN = (label.fields.field.(@name == 'CATALOG_PART_NUMBER').@data);

if (ITEM.isalphanum != 1 & ITEM !=""   )
  {
  label.fields.field.(@name == 'ITEM_NBR').@data = ITEM;
  }
else
  {
   if (CPN.isalphanum != 1 & CPN !=""   )
        {  
          label.fields.field.(@name == 'ITEM_NBR').@data = CPN;
        }
     else
        {  
          label.fields.field.(@name == 'ITEM_NBR').@data = CIN;
        }
   }