For example, I send alot of items with a tracking number.
In the dispatch notification I can already insert field [tracking_id].
However, I like to include text to inform buyer of how they can track it, web site to go to etc.
For items that do not have a tracking number it is probably confusing to the buyer (blank field) with superflous text and looks like a canned mail.
So in the message do something like
if isset('tracking_id') {
This package can be tracked using tracking number [tracking_id] at www.royalmail.co.uk
}

This would be for both Message in Dispatch Notification & Receipt of Order Notification