Lars Martin Fosse wrote:

How does this jibe with the idea that the signs were used for trade? So
far, we seem to have the following suggestions as to the Harappan sign
function:

1) a proper writing system
2) a logographic system used for trade purposes
3) a sign system possibly used for personal identification
4) a mnemonic system where the sign has the value of a set narrative

More than one of these functions may of course apply at the same time. But
I find 4) dubious. If we discard 1), we are left with varieties of 2) and
3). Combinations of signs may of course carry standardized messages, but if
we were dealing with a mnemonic system as described by George, why would we
find the signs used for trading purposes in the assumed way? How much
semantic/semiotic load can a sign have at the same time? Would a sign be
used as a shipment tag for customs purposes one day and as a symbol for a
standardized narrative on the next day?

Why not?  What are seals normally used for?  To provide a guarantee of authenticity or identification and perhaps means of detecting tampering, which would be most useful in trade and government.  To provide authenticity, it has to bear an emblem, perhaps with "script" as well, that identifies the source/sealer uniquely.  Hence, many elements of what is inscribed on seals are likely to be unique.

But identity can carry a message.  A Christian of the present era in the United States, in setting up an e-business web-site, might place on his "home" page not only his business's logo, but also an outline of a fish, an ancient Christian symbol of the faith, said to have been used for identifying one to another without attracting hostile notice.  A Christian from an older era might have put a cross on his seal.   Another may have been named by his parents after Jesus or another religious figure:  Christian Brown, Jesus Alou, Michael Jones, Mary Johnson.  Each collection of self-identifying symbols and signs thus may include symbols which, in themselves, imply a whole literature and religion, yet in this instance would simply serve as part of the identifying emblem/motto/address, etc.

David