Beyond what is found in other excellent references, a better understanding of S::STARTUP would be helpful with possibly a brief history on its usage in older releases of AutoCAD and also its relevance and unique fit as a customization tool in 2018.
P.S.: With extra time available now after over twenty years of other tending to other priorities, a serious (and dedicated) study of all things LISP is my focus with enthusiasm and the reality that much work lies ahead: Better late than never.