The atomic number effect. The difference in Z of the element of interest and the average Z of the sample. As more energy is lost, less energy is available to ionize inner shells to produce characteristic x-rays. More of the beam energy will be lost with backscattering and less energy will be available to produce x-rays. R term varies from 0.5 for heavy elements to 1 for light elements.
R also depends on the overvoltage.
Overvoltage is the ratio of the beam energy in keV over the critical ionization energy for the x-ray line of interest, as the overvoltage decreases, R will approach 1. This is dependent upon the path length of the x-ray as it leaves the sample and the mass absorption coefficient of the element of interest. A takes account of internal absorption of x-rays generated in the specimen. In practice a computer is attached to the SEM and peaks of various characteristic x-rays are displayed according to their energies. The peaks are then labeled by the computer according to their characteristic energies and each peak is matched to the element it represents.