According to the LEED v4: Building Design + Construction Guide, the 40/60 rule provides guidance for making a decision when several rating systems appear to be appropriate for a project. To use this rule, first assign a rating system to each square foot or square meter of the building, and then choose the most appropriate rating system based on the resulting percentages. The entire gross floor area of a LEED project must be certified under a single rating system and is subject to all prerequisites and attempted credits in that rating system, regardless of mixed construction or space usage type. If a rating system is appropriate for less than 40% of the gross floor area of a LEED project building or space, then that rating system should not be used. If a rating system is appropriate for more than 60% of the gross floor area of a LEED project building or space, then that rating system should be used. If an appropriate rating system falls between 40% and 60% of the gross floor area, project teams must independently assess their situation and decide which rating system is most applicable1. References: LEED v4: Building Design + Construction Guide, LEED Rating System Selection Guidance1