The concept behind Key Templates is to generate real time, original exercise traffic.

Too often operators are asked to just "make up" their own exercise traffic. At other times "canned" pre-written traffic is generated and distributed in advance. None of this actually tests an operator's ability to receive a message sight unseen, and pass it in a timely fashion in the desired mode (Voice, FLMSG, or Winlink).
Key Templates (with messages numbered 0-9) can be sent to operators in advance. When the exercise begins, Net Control (or whoever is in charge) announces the "key" to be used. Some examples:

The last digit of the year you were born
The last digit of your street address
The last digit of the year you graduated
The last digit of the year you got your ham radio license

Operators would then send the message that matches the key. At the same time Net Control would not know what numbered messages they would receive, so they would work in real-time as well.
Note: Yes, some operators would end up sending the same message (although their local information in the Header would likely be different). That is not the point. The important thing is that no one would know exactly what message would be sent in advance -- just like in a real world situation.

As an added bonus... If more traffic is needed, the same key can be used with a different form (ICS213, ARC 6409, Radiogram, etc...), or a different key can be released to generate another round of traffic using the same form. (Example: The original Key number, plus 1. If the original Key yielded a nine, go back to zero)

NOTE:
The PDF Files are "fillable", so they can be customized for your specific exercise.
The PNG Images are usually distributed to digital operators. This prevents them from simply "cut and pasting" the form's information when passing digital traffic. 

Feel free to use the forms in this section for ideas or as they are written.
Have fun!
73
Tim
NJ2N