the below word is form help document.
:

Tip

You can output the object ID and/or primitive IDs to deep raster planes, and then use them to easily select objects or primitives in the compositor.

To output the object or primitive ID, set up the following parameters:

* Variable name = Op_Id (or Prim_Id)
* File/Plane name = Op_Id (or Prim_Id)
* Channel depth, Variable type = Floating point
* Sample filter = closest surface
* Pixel filter = minmax idcover (a special filtering mode for ID channels)


the question is :

how and where to set up this parameters?

thanks

they go into the arbouts of a mantraROP

see this thread for additional information - pros and cons:
http://www.sidefx.com/index.php?option=com_forum&Itemid=172&page=viewtopic&t=11229&highlight=object [sidefx.com]