Hey Frank!
What makes it a Radar is that if you start reading the serial number starting from the front of the serial or the back it is still the same serial number.
Here are 3 common examples of 3-Digit Radar Notes:
1233321, 1213121, 1123211
(Your Note is the same as the first example)
Now here is another Radar note but being a 4-Digit Radar
1234321
What makes the difference between the category of radar notes is how many different digits the first 4 numbers are composed of (First set of examples there are only a total of 3 DIFFERENT Digits thus a 3-Digit Radar, the second example there are 4 DIFFERENT digits thus a 4-Digit Radar).
So now a 2-Digit Radar would only be composed of 2-Digits
(1112111, 1122211, 1222221)
As you can see, the first 4 Digits of the serial number are only composed of 2 different digits.
But if you look, Additionally to this they are still Radar notes (Same serial reads forwards and backwards the same).
Some Collectors collect 2-Digit Notes (Entire serial is only 2-Digits but does not meet the criteria for a Radar Note)
Example:
1112121, 1122221, 1222222 (You get the idea)
I hope this helps you understand. I do suggest buying a copy of the Charlton Catalog as it also has in great detail all about radar notes and other "Special" Serial numbers at the back of the book.