file can tuck this information away into a file UU/cc.cbu. Then,
after Configure prompts the user for the C compiler, it will load in
and run the UU/cc.cbu "call-back" unit. See hints/solaris_2.sh for an
-example.
+example. Some callbacks exist for other variables than cc, such as for
+uselongdouble. At the present time, these callbacks are only called if the
+variable in question is defined; however, this may change, so the scheme in
+hints/solaris_2.sh of checking to see if uselongdouble is defined is a good
+idea.
+
+=item Call status
+
+Call-backs are only called always, even if the value for the call-back is
+uset: UU/usethreads.cbu is called when Configure is about to deal with
+threads. All created call-backs from hints should thus check the status
+of the variable, and act upon it.
=item Future status