[plt-scheme] A question about external representation

Today I worked on a problem that involved converting a 'cond'
expression to 'case.

I was checking for 'value in the cond expression, but in the case I
should have been checking for unquoted value.

For example:

(define test2
  (λ (arg)
    (case arg
      ([test2] "quote test2")
      (['test2] "quote quote test2")
      (else #f))))

> (test2 'test2)
> (test2 ''test2)

Reading the documentation for case it says that each datum checked for
equality is "an external representation" of some object.

My question is, when we type (quote value), is the external
representation whatever shows up in the repl when that is evaluated?

What is external representation?