Undefined in LISP

In reading McCarthy's papers, there are things that struck me:

0. The only purpose of NIL is to mark the end of a list. When
representing falsity, McCarthy uses the symbol F .

1. There are some references to an expression being undefined.

I would like to know your thoughts, about what being undefined actually
is. Should an undefined expression be represented by a symbol, or should
an undefined expression be silent (without returning a symbol)?

--
The pioneers of a warless world are the youth that
refuse military service. ~ Albert Einstein