Additionl highlighter

Well, I know that maybe this is not the right forum to put in this question; I already sent it in the module design forum, but the real question is still unanswered.
The problem is : how the addition highligther function works ? I tried in several ways, using different kind of properties, but without any success. I resolved the problem of highlighting units using transparent layers, but this was not easy to handle and some problems are still remaining.
Your help would be very useful in understanding how this function really works.
Many thanks in advance for your cooperation on that matter.