[Boost-users] [test] CLOSE and CLOSE_FRACTION tolerance

I am trying to understand the tolerance value passed as the last parameter to the BOOST_XXX_CLOSE and BOOST_XXX_CLOSE_FRACTION macros. The doc says that for the former the value is percentage units while for the latter it is the fraction of absolute values being compared. But in each case there are two values, not just one. Which of the two values are used to determine the tolerance amount in each case ? If both values are being used in each case, how is the tolerance amount computed from the tolerance value passed ?