The idea of equivalent lateral force method is to distribute part of the seismic force (base shear) to every floor, which are able to transfer lateral loads.
As a result of this method, the static forces are generated and applied to rigid (or semi-rigid) diaphragms or vertical elements (columns, wall), which can carry calculated forces.
Every code proposes specific limitations of using such method. Most common limitations are structure regularity and its height.
Mass of the storey includes added masses and dynamic masses from converted loads.
Diaphragms assure proportional distribution of seismic loads on vertical elements. Generated force should be applied to the center of mass of the diaphragm . Diaphragm or panel not positioned at the plane of storey top does not carry seismic force.
In case of lack of diaphragms masses of nodes lying at the plane of storey top (floor plane) are taken into account. Then the proportional to mass force distribution has to be performed.