In this combo you can select the Mask from the list of predefined or you can type your own mask.
A Mask is a validation string that define the acceptable field input value for each characters. The mask determine which characters are valid in input for each characters position and define also the max acceptable length of the string.
The mask has this special value:
A - Allows only the UNICODE alphabetic characters (upper or lower-case)
W - Allow all the all the alphabetic characters plus the numbers
0 - Allows only number (0-9)
? - Allow any characters
If you enter other character (other than the above) it will be inserted into the field data

For example:
AA-000-AA--> the user can enter 2 letters 3 number 2 letters
AA-???-AA --> the user can enter 2 letters 3 any chars 2 letters