The #Insert# macro allows you to insert a line of data from an external file. If you create a folder in "ArmTracker\Email" called "Extra" and place a text file called "Coupons.txt", you could insert a line from this file be specifying:
#Insert#Extra\Coupons.txt#
This allows you to create a file with lines of data (invoice numbers, coupon numbers, etc) and grab a line of data from this file every time the #Insert# macro is called.
Once a line of data is used from the specified file, it is marked with a ~ symbol and that line is not used again. This allows every inserted line to be unique.
Note: Always make sure you have enough lines of data in your external file to corrispond with the number of customers you have selected to have data inserted into.