Open Message Model consumer applications can encode and send data for any item via Post messages on the item’s Login stream. This is known as off-stream posting because items are posted without using that item’s dedicated stream. Posting an item on its own dedicated stream is referred to as on-stream posting.
When an application is posting off-stream, the PostMsg requires Name and ServiceId information. For more details on posting, refer to the Enterprise Message API C++ Edition Developers Guide.