A generalized variable or place form is one of the many
places in Lisp memory where values can be stored using the setf
macro (see The setf Macro). The simplest place
form is a regular Lisp variable. But the CARs and CDRs of
lists, elements of arrays, properties of symbols, and many other
locations are also places where Lisp values get stored.
Generalized variables are analogous to lvalues in the C
language, where ‘x = a[i]’ gets an element from an array
and ‘a[i] = x’ stores an element using the same notation.
Just as certain forms like a[i] can be lvalues in C, there
is a set of forms that can be generalized variables in Lisp.