This question is answered best by Google:
Being specific will help improve your data quality and in turn ensures that offers are presented in the right category. When viewing the taxonomy, try to navigate to the most specific category that still covers all the products being submitted. For instance, if you are submitting MP3 player chargers use the category MP3 Player Chargers. Conversely, if you are submitting multiple types of chargers use the category Chargers.
More information, including a link to Google’s taxonomy can be found here: Categorize your products