Assembly Code.

Assembly Code in Revit

http://forums.autodesk.com/t5/Autodesk-Revit-Architecture/Assembly-Code/m-p/1863367

http://www.revitcity.com/forums.php?action=viewthread&thread_id=13315

http://www.innovaya.com/prod_revit.htm

http://docs.autodesk.com/REVIT/2011/ENU/filesUsersGuide/WSfacf1429558a55debd637aff4f2f7abe-7f4c.htm