Yes. The tier methodology applies to each unit and to each fuel combusted in a unit. Therefore, you can use different tiers for different units and different tiers for different fuels in a co-fired unit (providing that an allowable tier method is used in each situation). The exception is if you use common pipe provisions for multiple units. In this case, you must use the highest tier method that applies to any unit within the aggregated group for which you use the common pipe provision.