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The self weight of the walls are multiplied by a height equal to 1/2 the wall height on the level below the diaphragm plus 1/2 the height of the wall above the diaphragm. This creates line load building masses – i.e., the area density in the vertical plane is converted to a line load in the horizontal plane. Note that the first level building mass is derived from the upper half of the wall only; the lower half is assumed to go directly into the foundation below without contributing to shear force in the diaphragm above.
The length and location of the line load are exactly the same as those of the wall. Depending on the orientation of the wall relative to the seismic force, the building masses will turn into seismic point loads or seismic line loads on the diaphragm.