Resolution:
The key/value mechanism is used when the request and the reply use independent subjects.  To coordinate replies with the corresponding request , a unique value is sent out with the request and returned with the reply.

The key fields (request & reply) are the message tags that hold this unique key value.  The key value is generated by the TIB/Integration Manager engine and is usually a LARGE number (8 bytes).  Some systems (e.g. SQL databases) can't handle 8 byte numbers, so an option to send the key value as a string is provided

Refer to example RequestReplyWithKey in IM_Examples.zip under the samples directory.