The text in 3.3 and 5.8.1.1 has been modified to highlight these facts,
with some minor changes elsewhere. Specifically, in 3.3:

   o  The format of the information that describes the path that the
      signalling should take, the Message Routing Information (MRI).
      For the path-coupled MRM, this is just the Flow Identifier (see
      Section 5.8.1.1) and some additional control information.
      Specifically, the MRI always includes a flag to distinguish
      between the two directions that signalling messages can take,
      denoted 'upstream' and 'downstream', and this flag would be
      inverted between a signalling message and its response.

and in 5.8.1.1:

                    ... A further flag in the MRI defines the
   direction of this particular signalling message relative to the flow
   described.  Therefore, for example, the flag would be inverted
   between a Query and Response message referring to the same flow.

It needs to be clear that the MRI is a description of 
- how to route a signalling message with respect to a flow
NOT
- a description of the flow itself.
Therefore, different signalling messages about the same flow will have different
values of the D flag, depending on the signalling message direction. For
example, the D flag will be inverted between Queries and Responses.