We've received some feedback from Google Product Search merchants asking for more direction on which values to include for the product_type attribute. We listened to your feedback, and today we're unveiling our new categorization system that you can use for describing the product type of your items. Product_type is an optional attribute that you can use to indicate the category (or categories) of the product being sold. We recommend that you use the category paths and values from our new product taxonomy.
The Google product taxonomy is a tree of categories that describe product families, with verticals (Electronics, Home & Garden, etc.) at the highest level, followed by more specific product families or products within these broad categories (such as Electronics > Audio > Audio Players & Recorders > MP3 Players).