This is a somewhat tricky property; some further explanation is in order. Let's take a look at an actual Microsoft product key as an example:

 

The key "CB48T - GGBCX - H269K - C9W64 - X2RWD" would be represented in the SerialNumberTemplate property as "^^%#^ - ^^^^^ - ^%%%^ - ^%^%% - ^%^^^" - the "^" characters represent placeholders for letters, the "#" character represents a number that is ignored, and the "%" character represents a number that will be evaluated by the validation algorithm. So in this case, if you add together the numbers represented by "%" and ignore the number represented by "#" you get 42; dividing this by 7 leaves a remainder of 0 so the validation succeeds.

 

If you were to use this same template for your product key, you would need to generate a series of keys to provide to your customers when you distribute your program, randomizing the placeholder characters, and also randomizing the numbers but making sure that the numbers represented by "%" are divisible by 7.

 

Note that there is a seemingly arbitrary restriction in that each time you change the editable character a new text box is created, so the above example would create 12 textboxes, not 5 as in a Microsoft product key. To get around this restriction, make sure that only one grouping contains the numbers that you will validate, and use the "?" character in all of the other fields: "????? - %%%%% - ????? - ????? - ?????". You could still use a mix of numbers and letters in your keys, but the second box would always contain numbers - i.e. "BC45T - 340707 - 1XFFE - EFT22 - 1XYZ3".

It appears that entering 7 or 0 for all characters in "????? - %%%%% - ????? - ????? - ?????" will validate, not what we want. On the other hand This "<??^? - %### - ^?^ - #%%% - ^??^>" will not, which is what we want. We cant mix ? or ^ with % or # or a new box is created, (%##^) would make 2 boxes and (%#^#) would make 3 boxes, therefore the reason for mixing ? and ^ is that they combine to form a box which will except numbers or letters but the letters must be in the right place when entered by the user making it much harder to find the combination.

It's very-very easy to break this "encryption".
No matter what key is.
Just start to enter a character (let's say "G"). If you receive the message "Unacceptable character. You can type only digits here" - type all the "7" in this text box.
In the next text box start typing "G" again.
That's it! The key will be accepted!