Constants in concrete5 5.7

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I kind of unserstand the new config system that is in concrete/config/app and application/config/app and also concrete/config/concrete.php

I saw in the register single page controlller :
if (defined('EMAIL_ADDRESS_VALIDATE')) {
                        $mh->from(EMAIL_ADDRESS_VALIDATE,  t('Validate Email Address'));
                    }


where does EMAIL_ADDRESS_VALIDATE get set? and other constants used in concrete5 5.7? thanks for any input on this issue!!