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Thanks for asking. An input mask forces users to enter data in a specific format. Have a look at this jotform. Phone Number 1 doesn't have any mask while Phone Number 2 does. Just type any random number in each and note the difference. See how the parentheses, space and dash are inserted by default in the masked field? Moreover, only a limited number of characters can be entered there. If you were to clone the form, click the masked field and then click the Input Mask button, you would see that the # sign was used to allow only numbers to be entered. Also, the Fill Mask Validation box is checked. This triggers an error message and prevents the form from being submitted if all of the characters are not entered: