In local, open discharge systems, the force is applied at the discharge of the piping. See API 520 diagrams. This simplified method represents the steady state condition, some time after the valve has opened, and assumes that the piping lengths are sufficiently short enough that the pressure wave cancels itself early enough to be insignificant by the time the final discharge occurs. ASME B31.1 has guidance regarding DLF here.
In closed discharge systems, a force will occur at every bend, but the magnitude of that force will be a fraction of the previously calculated force, which considers opening time for the valve, the speed of sound of the fluid, and the lengths between bends upstream of each bend. All forces are applied in the opposite direction of flow out of each element. (If the elbow is traveling +Y and turns +X, force is applied in the -X direction). Whether you apply the force to the "upstream" bend or the "downstream" bend is immaterial, as forces applied axially to a pipe in CAESAR will be transmitted perfectly through the "thin stick." This methodology is spelled out in L.C. Peng's "Pipe Stress Analysis." Within, he specifies considering only the elbows within total length of the PSV equal to speed of sound times valve opening time.
In remote, open discharge systems (quasi-open/closed), apply both logic: small forces on the bends, large force on the discharge. You should calculate pressure losses and velocity increases in the line for the discharge.
In all cases, the magnitude of the force is (more or less) mass flowrate times velocity plus pressure times area. Typically, this is taken at the valve as a matter of conservatism and simplicity.