  The changing nature of power systems dynamics is challenging present
practices related to modeling and study of system-level dynamic behavior. While
developing new techniques and models to handle the new modeling requirements,
it is also critical to review some of the terminology used to describe existing
simulation approaches and the embedded assumptions. This paper provides a
first-principles review of the simplifications and transformation commonly used
in the formulation of time-domain simulation models. It introduces a taxonomy
and classification of time-domain simulation models depending on their
frequency bandwidth, network representation, and software availability.
Furthermore, it focuses on the fundamental aspects of averaging techniques, and
model reduction approaches that result in modeling choices, and discusses the
associated challenges and opportunities of applying these methods in systems
with large shares of Inverter Based Resources (IBRs). The paper concludes with
an illustrative simulation that compares the trajectories of an IBR-dominated
system.
