IS_BUILTIN vs USE_BUILTIN

I'm finding confusion in the builtin scheme, specifically looking at
expat.

There is

  IS_BUILTIN.expat
  USE_BUILTIN.expat

As I understand from mk/, IS_BUILTIN is about whether there is some
implementation of expat in the base system, *regardless of whether it is
good enough*.

Then, USE_BUILTIN should be set to yes if expat's builtin.mk decides
that the base version is good enough.

However textproc/expat/builtin.mk seems to check if IS_BUILTIN is not
set, and then set it conditionally.

My question is:

  Is IS_BUILTIN supposed to include "good enough" and USE_BUILTIN
  supposed to include "PREFER machinery says use it"?

or is the good enough check supposed to be in IS_BUILTIN?