S::STARTUP
Short answer
A special LISP function name AutoCAD calls automatically after any drawing finishes loading.
(defun S::STARTUP () …) is a reserved function name. If AutoCAD finds a function called S::STARTUP defined when a drawing finishes loading, it calls it once with no arguments. This is the supported way to run per-drawing initialization code from acaddoc.lsp or an MNL file. If multiple LISP files each define S::STARTUP, only the last one loaded wins — the safe pattern is to append to S::STARTUP rather than redefine it.