[boost] [test] Confusion about BOOST_CHECK_CLOSE

I have re-read everything I can find about BOOST_CHECK_CLOSE, and the doc on floating point comparison algorithms (i.e.close_at_tolerance). However, I am still confused about how to use BOOST_CHECK_CLOSE. Could we have some clear examples of comparisons that pass/fail with different tolerance settings, and some explanation as to how changing the tolerance may affect the test? Currently, I have a test, using BOOST_CHECK_CLOSE() which passes, no matter how big/small I set the tolerance; yet the values are different. Thanks!