For a bulk SMS notification, I want to be able to include personal information into the message.
The way in which characteristics are set at the message level, does not provide the ability to define certain characteristics on a per user basis.
Eg Hi $FirstName, can I offer you a great deal on the $Product for a price today of \$$Amount. ....
in this case $Amount and $Product could be set in the message whereas $FirstName needs to be drawn from the recipient.
Also note, I have not found (newbee problem maybe) where the rules around substitution are defined, eg how as in the above example can i escape what appears to be the use of a $ symbol to define a substibution paramater name with the literal $.
I gues the question also goes to the lifecycle of creating a message and (possbily later) adding in paramter values.
The API is not prescriptive regarding the content of the communication message, and the $ symbol that you see in the examples is just illustrative, not normative.
The generation of content based on templates and replaceable parameters would be done prior to invoking the relevant end-points in this API.
Hope it helps