My understanding is the ELF is a simplification of a dynamic analysis for a 'typical' building. Usually defined as having 90% of the building mass participate in the first mode. If for one reason or another (mass irregularity, geometric irregularity, etc) you have less than 90% of your mass participating in the first mode, you need to do a modal response spectrum analysis and properly combine the effects of several modes to accurately capture your structures dynamic response.
Time history analysis is usually done on a case by case basis when you have site specific seismic information as a way of hopefully achieving reduced seismic forces as opposed to the traditional prescriptive methods of Ch12 in ASCE 7-10.
