design and construction – for commercial and retail interiors Green building operations and maintenance Green neighborhood development Green home design and construction LEED v3 aligned credits across all LEED rating systems, weighted by environmental priority. It reflects a continuous development process, with a revised third-party certification program and online resources. Under LEED 2009, an evaluated project scores points to a possible maximum of 100 across six categories: sustainable sites, water efficiency, energy and atmosphere, materials and resources, indoor environment quality (IEQ) and design innovation. Each of these categories also includes mandatory requirements, which receive no points. Up to 10 additional points may be earned: 4 for regional priority credits and 6 for innovation in design. Additional performance categories for residences (LEED for Homes) recognize the importance of transportation access, open space, and outdoor physical activity outdoors, and the need for buildings and settlements to educate occupants. Buildings can qualify for four levels of certification: Certified: 40–49 points Silver: 50–59 points Gold: 60–79 points Platinum: 80 points and above The aim of LEED 2009 is to allocate points "based on the potential environmental impacts and human benefits of each credit". These are weighed using the environmental impact categories of the EPA's Tools for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI) and the environmental-impact weighting scheme developed by the National Institute of Standards and Technology (NIST). Prior to LEED 2009 evaluation and certification, a building must comply with environmental laws and regulations, occupancy scenarios, building permanence and pre-rating completion, site boundaries and area-to-site ratios. Its owner must share data on the building's energy and water use for five years after occupancy (for new construction) or date of certification (for existing buildings). The credit weighting process has the following steps: First, a collection of reference buildings are assessed to estimate the environmental impacts of similar buildings. NIST weightings are then applied to judge the relative importance of these impacts in each category. Data regarding actual impacts on environmental and human health are then used to assign points to individual categories and measures. This system results in a weighted average for each rating scheme based upon actual impacts and the relative importance of those impacts to human health and environmental quality. The LEED council also appears to have assigned credit and measured weighting based upon the market implications of point allocation. From 2010, buildings can use carbon offsets to achieve