U = uppercase letter 9 = any digit Z = any letter L = lowercase letter * = any letter or digit
This information is also shown as helptext in the modeler when you focus on the input mask field in the behavior tabpage of a textbox.
9 = any digit U = uppercase letter
In addition to the previous answers:
The input mask limits what the user can enter in the text box. A '9' means any digit, 'Z' means any letter, 'U' an upper-case, 'L' a lower-case letter and '*' a letter or a digit. Other characters will be taken literally. For example, the input mask 99-LLL-9999 matches 24-apr-2008.
I would also like refer to this Question & Answers