Comp a add B
 

For XP

<TransitionsAndEffects Version="1.0">
<Transitions>
<TransitionDLL guid="{9A43A844-0831-11D1-817F-0000F87557DB}" >
<Transition name="Comp A add B" iconid="0" comment="DXCOMPFUNC_A_ADD_B">
<Param name="Function" value="9" />
</Transition>
</TransitionDLL>
</Transitions>
</TransitionsAndEffects>

 

For Vista

 

Check out this  link for the details http://movies.blainesville.com/2007/07/compositing-and-translucency-now.html