Industry ratios will give you a benchmark for spending for an industry. This will allow you to see what percentage companies in a specific industry are spending on different business expenditures.
NAICS (North American Industry Classification System) is used by the government to gather and organize industry statistics. it is also used by multiple resources to organize data--like the ratio books above.
SIC (Standard Industrial Classification) is no longer active but it still used by some resources, like Industry Norms, as a way to organize information.