When the orientation of a block is identical to that of its neighbour, the smsg array consists of the unknowns numbered in the increasing coordinate (i,j,k) directions which is consistent consistent with the numbering of grid points of the block the unknowns are coming from. In such cases we have iorient = 0 (see itopol ) and no special precautions (sign changes of unknowns for example) are necessary to transport unknowns from a block to its neighbour.
In a general situation however, the orientations of blocks may be chosen independently of each other. This means that we must reverse the order of copying or change some signs of unknowns when copying from one block to its neighbour. Although the above procedure still gives a unique representation of unknowns for itrans = 1 and itrans > 2 it is not unique for itrans = 2. In the latter case, the unknowns reside on the interface and belong to both blocks and therefore in case of different orientations of subdomains this gives problems.
A solution to this problem is the following. Denote a
subface k of side j of block i by 
. If two
blocks share a subface, then we have subface 
identical
to 
.
The ambiguity in the representation of a multi-block submessage
is now resolved simply be chosing the representation based on the
block for which
is the smallest.
This procedure gives a unique representation for all transported
quantities itrans.