For ASHRAE 62.2-2016 you may claim an infiltration credit for horizontally attached dwelling units in a multifamily building, but not for dwelling units that are one above the other (stacked). For ASHRAE 62.2 versions before 2016, you may not claim an infiltration credit for ANY dwelling unit in a multifamily building.
The horizontally attached infiltration credit is a partial credit based on the ratio of the area of the exterior surfaces NOT connected to other dwelling units to the total surface area. For example, if the exterior surface area not connected to other dwelling units is 2400 square feet and the surface area common to the adjacent unit(s) is 600 square feet, the total surface area is 3000 square feet. So you may claim 80% of the full infiltration credit (2400/3000 = 0.8). If the blower door test result for this horizontally attached dwelling unit it 1600 CFM50, the allowed credit is 1600 x 0.8 = 1280 CFM50.