[pro] DEFVARs combined service

"DEFVAR and DEFPARAMETER do the combined service of - in some implementations, recording the "definitional home" of the variable - declaring the indicated variable special in compiler and runtime - assigning the variable (either conditionally, as in DEFVAR, which assigns only in the case that the variable is unbound, or unconditionally, as in DEFPARAMETER). " K. Pitman, comp.lang.lisp, 26. April 2001. What "recording the definitional home" means? Why "in some implementations?"