Determine where an outbound should go to. For every outbound Idoc the receiver needs to be filled, if it's not filled, the CDM could still fill it for you... The absolute strength behind this concept is that more than one receivers could be set up for an Idoc, effectively creating an Idoc per receiver. Via transaction BD64 - Maintain Customer Distribution Model, settings per partner profile - message type combination can be done, effectively allowing the Idoc that is created without receiver to be send out (with receiver).
How should this be used ? Remarkably simple: use the Change pointer setup to generate your (receiverless) Idocs, or build one yourself, using the MASTER_IDOC_DISTRIBUTE function module, which will return a list of actual Communication Idocs. A communication Idoc is an actual Idoc that can be found on the database (table EDIDC etc.) The master idoc is a fictional Idoc: it is handed to MASTER_IDOC_DISTRIBUTE and the CDM may well create several communication Idocs for it, but a master Idoc is not actually stored. This does imply that an Idoc created without receiver and which does not has CDM settings in place will result in nothing at all (!)