Google Product Search has always had “out-of-the-box” support for a number of product attributes.  You can find the supported attribute list in their feed spec documentation.  While this list of supported attributes is robust (and has gotten more lengthy over the years), Google still allows merchants to submit “custom attributes” if they have additional, structured product data to provide.  For example, a power tool merchant may store information about the power of each of their handheld tools (ie: 12v, 14v, 18v).  In this case the retailer could submit these values to Google in a custom attribute field.  The benefit of doing this is still somewhat TBD and depends on the product category the merchant is participating in.  Sometimes Google could display your custom attribute as a filter, or they may index your attribute values so that they are search-able (ie:  if someone searched for “18v handheld drill” your listings would get a ranking boost)… but all of this is part of Google’s secret sauce, so merchants really just need to test various strategies.  Custom attributes are provided in the following format:  “c:attribute_name:attribute_type”.  Seven attribute types are supported and they include types like:  integer, datetime, string…  in the previous example, the power tool retailer could provide a field named:   “c:power:integer” and include values like “14″ or “18″ in that field.

Knowing all of the above, we learned something new about custom attributes recently.   If you include a custom attribute in your file with an attribute name that matches a supported attribute, then your feed will fail!  We aren’t sure if this occurs 100% of the time but it recently occurred for a SingleFeed customer and Google confirmed the problem.  As an example, if you include an attribute like “c:color:string”, your feed could fail.  This is because “color” is a supported Google attribute.  Whats even more perplexing is that, when feeds fail for this reason, there is no appropriate error messaging identifying the problem… the ambiguous “0 items of X items were inserted” message was given.  Obviously, in an ideal world, Google would recognize these custom attributes and simply treat them as supported attributes.  The second best scenario would be for them to return a feed processing error saying “you have custom attributes that should be regular attributes… please resubmit”.  Unfortunately, what’s currently occurring is the worst case scenario:  the feed fails with little to no feedback.  While this may be an edge case, we encountered it recently and so wanted to make sure you keep it in mind!