+&half_space
+&ditem(sub_id)
+This value is the subscription ID. It, in combination with the message type is used
+by rmr to determine the target endpoint when sending a message.
+If the application to application protocol does not warrant the use of a subscription
+ID, the RMR constant RMR_VOID_SUBID should be placed in this field.
+When an application is forwarding or returning a buffer to the sender, it is the
+application's responsibility to set/reset this value.
+
+&half_space
+&ditem(tp_state)
+For C applications making use of RMR, the state of a transport based failure will
+often be available via &cw(errno.)
+However, some wrapper environments may not have direct access to the C-lib &cw(errno)
+value.
+RMR send and receive operations will place the current value of &cw(errno) into this
+field which should make it available to wrapper functions.
+User applications are strongly cautioned against relying on the value of errno as
+some transport mechanisms may not set this value on all calls.
+This value should also be ignored any time the message status is &cw(RMR_OK.)
+