The detail and thoroughness of the attribute values system of a taxonomy.
Some projects will place a lot of value on a deep schema if their goal is to capture as much information about their products as possible. Industry leaders might work to develop definitive compilations of carefully workshopped attributes with standardized lists of values and attribute dependencies in order to deepen their pool of readily available data for each SKU. However, other clients working on a deadline might only require a thin schema of 3 to 5 attributes. Each project has different requirements, so the best schema depth will depend on the nature and focus of each individual taxonomy.