For example, to allow your MantisBT installation to be viewed in a frame in IE6 when the frameset is not at the same hostname as the MantisBT install, you need to add a P3P header. You could try something like
$g_custom_headers = array( 'P3P: CP="CUR ADM"' );
in your config file, but make sure to check that your policy actually matches with what you are promising. See
MSDN for more information.
Even though it is not recommended, you could also use this setting to disable previously sent headers. For example, assuming you didn't want to benefit from Content Security Policy (CSP), you could set:
$g_custom_headers = array( 'Content-Security-Policy:' );