You will notice that the program does not prevent you from assigning an alphanumeric field to track 2 or 3. You may have an alphanumeric field which only contains numbers, for example, so there is no reason to exclude it from those tracks.
Nor is any check of the suitability of any of the data made at this time. This is because the data will almost certainly vary from badge to badge. The validity of the data is actually checked when the badge is encoded during the printing process. This is when the data from a specific badge becomes known and can be checked.